Polarization-modulating optical element

ABSTRACT

A microlithography optical system includes a projection objective and an illumination system that includes a temperature compensated polarization-modulating optical element. The temperature compensated polarization-modulating optical element includes a first polarization-modulating optical element of optically active material, the first polarization-modulating optical element having a first specific rotation with a sign. The temperature compensated polarization-modulating optical element includes also includes a second polarization-modulating optical element of optically active material, the second polarization-modulating optical element having a second specific rotation with a sign opposite to the sign of the first specific rotation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/201,767, Aug. 29, 2008, which is a continuation application of U.S.application Ser. No. 11/440,475, filed May 25, 2006, which is aContinuation-In-Part of, and claims priority under 35 U.S.C. §120 to,International Application PCT/EP2005/000320, having an internationalfiling date of Jan. 14, 2005, which claimed the benefit of U.S. Ser. No.60/537,327, filed on Jan. 16, 2004. This application also claims thebenefit under 35 U.S.C. §119 of U.S. Ser. No. 60/684,607, filed on May25, 2005. Each of these applications is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to an optical element that affects thepolarization of light rays. The optical element has a thickness profileand consists of or comprises an optically active crystal with an opticalaxis.

BACKGROUND

In the continuing effort to achieve structures of finer resolution inthe field of microlithography, there is a parallel pursuit ofsubstantially three guiding concepts. The first of these is to provideprojection objectives of very high numerical aperture. Second is theconstant trend towards shorter wavelengths, for example 248 nm, 193 nm,or 157 nm. Finally, there is the concept of increasing the achievableresolution by introducing an immersion medium of a high refractive indexinto the space between the last optical element of the projectionobjective and the light-sensitive substrate. The latter technique isreferred to as immersion lithography.

In an optical system that is illuminated with light of a definedpolarization, the s- and p-component of the electrical field vector, inaccordance with Fresnel's equations, are subject to respectivelydifferent degrees of reflection and refraction at the interface of twomedia with different refractive indices. In this context andhereinafter, the polarization component that oscillates parallel to theplane of incidence of a light ray is referred to as p-component, whilethe polarization component that oscillates perpendicular to the plane ofincidence of a light ray is referred to as s-component. The differentdegrees of reflection and refraction that occur in the s-component incomparison to the p-component have a significant detrimental effect onthe imaging process.

This problem can be avoided with a special distribution of thepolarization where the planes of oscillation of the electrical fieldvectors of individual linearly polarized light rays in a pupil plane ofthe optical system have an approximately radial orientation relative tothe optical axis. A polarization distribution of this kind willhereinafter be referred to as radial polarization. If a bundle of lightrays that are radially polarized in accordance with the foregoingdefinition meets an interface between two media of different refractiveindices in a field plane of an objective, only the p-component of theelectrical field vector will be present, so that the aforementioneddetrimental effect on the imaging quality is reduced considerably.

In analogy to the foregoing concept, one could also choose apolarization distribution where the planes of oscillation of theelectrical field vectors of individual linearly polarized light rays ina pupil plane of the system have an orientation that is perpendicular tothe radius originating from the optical axis. A polarizationdistribution of this type will hereinafter be referred to as tangentialpolarization. If a bundle of light rays that are tangentially polarizedin accordance with this definition meets an interface between two mediaof different refractive indices, only the s-component of the electricalfield vector will be present so that, as in the preceding case, therewill be uniformity in the reflection and refraction occurring in a fieldplane.

Providing an illumination with either tangential or radial polarizationin a pupil plane is of high importance in particular when putting theaforementioned concept of immersion lithography into practice, becauseof the considerable negative effects on the state of polarization thatare to be expected based on the differences in the refractive index andthe strongly oblique angles of incidence at the respective interfacesfrom the last optical element of the projection objective to theimmersion medium and from the immersion medium to the coatedlight-sensitive substrate.

SUMMARY

In one aspect, the invention generally features a microlithographyoptical system that includes an illumination system, a projectionobjective and a temperature compensated polarization-modulating opticalelement in the illumination system. The temperature compensatedpolarization-modulating optical element includes a firstpolarization-modulating optical element that includes an opticallyactive material. The first polarization-modulating optical element has afirst specific rotation with a sign. The temperature compensatedpolarization-modulating optical element also includes a secondpolarization-modulating optical element comprising optically activematerial. The second polarization-modulating optical element has asecond specific rotation with a sign opposite to the sign of the firstspecific rotation.

In another aspect, the invention generally features using the systemdescribed in the preceding paragraph to manufacturing a micro-structuredsemiconductor component.

In a further aspect, the invention generally features an optical elementthat includes a temperature compensated polarization-modulating element.The temperature compensated polarization-modulating element includes afirst polarization-modulating optical element having a first thickness.The first polarization-modulating element includes an optically activematerial with a first specific rotation having a sign. The temperaturecompensated polarization-modulating element also includes a secondpolarization-modulating optical element having a second thicknessdifferent from said first thickness. The second polarization-modulatingoptical element includes an optically active material with a secondspecific rotation having a sign opposite the sign of the first specificrotation.

In an additional aspect, the invention generally a microlithographyoptical system that includes an illumination system, a projectionobjective and the optical element described in the preceding paragraph.The optical element is in the illumination system.

In another aspect, the invention generally features using the systemdescribed in the preceding paragraph to manufacturing a micro-structuredsemiconductor component.

In a further aspect, the invention generally features an optical systemthat has an optical axis. The optical system includes a first planeplate and a second plane plate. The first plane plate includes opticallyactive quartz, and the first plane plate has a first thickness in thedirection of the optical axis. The second plane plate includes opticallyactive quartz. The second plane plate has a second thickness in thedirection of the optical axis, and the second thickness being differentfrom the first thickness.

In an additional aspect, the invention generally features a system thatincludes an illumination system, a projection objective and an opticalsystem having an optical axis. The optical system includes a first planeplate and a second plane plate. The first plate includes opticallyactive quartz. The first plane plate has a first thickness in thedirection of the optical axis. The second plane plate includes opticallyactive quartz. The second plane plate has a second thickness in thedirection of the optical axis, and the second thickness being differentfrom the first thickness.

In one aspect, the invention generally features an optical element thatincludes a support plate comprising optically active material, and atleast two planar-parallel portions. Each of the at least twoplanar-parallel portions includes optically active material. When afirst linearly polarized light ray passes through the optical element, aplane of oscillation of the first linearly polarized light ray isrotated by a first angle. When a second linearly polarized light raypasses through the optical element, a plane of oscillation of the secondlinearly polarized light ray is rotated by a second angle different fromthe first angle.

In another aspect, the invention generally features a microlithographyoptical system that includes an illumination system, a projectionobjective and the optical element described in the preceding paragraph.The optical element is in the illumination system.

In a further aspect, the invention generally features using the systemdescribed in the preceding paragraph to manufacturing a micro-structuredsemiconductor component.

In an additional aspect, the invention generally features an opticalarrangement that includes a polarization-modulating optical element thatincludes a first optically active material having a first specificrotation with a sign. The polarization-modulating optical element has afirst optical axis, and the polarization-modulating element has a firstthickness profile that, as measured in the direction of the firstoptical axis, is variable. The optical arrangement also includes acompensation plate that includes a second optically active materialhaving a second specific rotation with a sign opposite the sign of thefirst specific rotation. The compensation plate has a second thicknessprofile configured so that, when radiation passes through the opticalarrangement, the compensation plate substantially compensates for angledeviations of the radiation that are caused by thepolarization-modulating optical element.

In another aspect, the invention generally features a microlithographyoptical system that includes an illumination system, a projectionobjective and the optical arrangement described in the precedingparagraph. The optical arrangement is in the illumination system.

In a further aspect, the invention generally features using the systemdescribed in the preceding paragraph to manufacturing a micro-structuredsemiconductor component.

In certain embodiments, a polarization-modulating optical element isprovided, which—with a minimum loss of intensity—affects thepolarization of light rays in such a way that from linearly polarizedlight with a first distribution of the directions of the oscillationplanes of individual light rays, the optical element generates linearlypolarized light with a second distribution of the directions of theoscillation planes of individual light rays.

In some embodiments, an optical system is provided that has improvedproperties of the polarization-modulating optical element regardingthermal stability of the second distribution of oscillation planes(polarization distribution), and/or minimized influence of additionaloptical elements in the optical system to the polarization distributionafter the light rays have passed these elements.

In some embodiments, a polarization-modulating optical element isprovided which consists of or comprises an optically active crystal andwhich is shaped with a thickness profile that varies in the directionsperpendicular to the optical axis.

A polarization-modulating optical element can have the effect that theplane of oscillation of a first linearly polarized light ray and theplane of oscillation of a second linearly polarized light ray arerotated, respectively, by a first and a second angle of rotation, withthe first angle of rotation being different from the second angle ofrotation. The polarization-modulating optical element can be made of anoptically active material.

In some embodiments, one or more of the following desirable features canbe provided.

In order to generate from linearly polarized light an arbitrarilyselected distribution of linearly polarized light rays with a minimumloss of intensity, an optically active crystal with an optical axis isused as raw material for the polarization-modulating optical element.The optical axis of a crystal, also referred to as axis of isotropy, isdefined by the property that there is only one velocity of lightpropagation associated with the direction of the optical axis. In otherwords, a light ray travelling in the direction of an optical axis is notsubject to a linear birefringence. The polarization-modulating opticalelement has a thickness profile that varies in the directionsperpendicular to the optical axis of the crystal. The term “linearpolarization distribution” in this context and hereinafter is used withthe meaning of a polarization distribution in which the individual lightrays are linearly polarized but the oscillation planes of the individualelectrical field vectors can be oriented in different directions.

If linearly polarized light traverses the polarization-modulatingoptical element along the optical axis of the crystal, the oscillationplane of the electrical field vector is rotated by an angle that isproportional to the distance travelled inside the crystal. The sense ofrotation, i.e., whether the oscillation plane is rotated clockwise orcounterclockwise, depends on the crystal material, for exampleright-handed quartz vs. left-handed quartz. The polarization plane isparallel to the respective directions of the polarization and thepropagation of the light ray. In order to produce an arbitrarilyselected distribution of the angles of rotation, it is advantageous ifthe thickness profile is designed so that the plane of oscillation of afirst linearly polarized light ray and the plane of oscillation of asecond linearly polarized light ray are rotated, respectively, by afirst and a second angle of rotation, with the first angle of rotationbeing different from the second angle of rotation. By shaping theelement with a specific thickness at each location, it is possible torealize arbitrarily selected angles of rotation for the oscillationplanes.

Different optically active materials have been found suitable dependenton the wavelength of the radiation being used, specifically quartz,TeO₂, and AgGaS₂.

In an advantageous embodiment, the polarization-modulating opticalelement has an element axis oriented in the same direction as theoptical axis of the crystal. In relation to the element axis, thethickness profile of the optical element is a function of the azimuthangle θ alone, with the azimuth angle θ being measured relative to areference axis that intersects the element axis at a right angle. With athickness profile according to this design, the thickness of the opticalelement is constant along a radius that intersects the element axis at aright angle and forms an azimuth angle θ with the reference axis.

In a further advantageous embodiment, an azimuthal section d(r=const.,θ)of the thickness profile d(r,θ) at a constant distance r from theelement axis is a linear function of the azimuth angle θ. In the idealcase, this azimuthal section has a discontinuity at the azimuth angleθ=0. The linear function d(r=const.,θ) at a constant distance r from theelement axis has a slope

${|m| = \frac{180{^\circ}}{{\alpha\pi}\; r}},$

wherein α stands for the specific rotation of the optically activecrystal. At the discontinuity location for θ=0, there is an abrupt stepin the thickness by an amount of 360°/α. The step at the discontinuitylocation can also be distributed over an azimuth angle range of a fewdegrees. However, this has the result of a non-optimized polarizationdistribution in the transition range.

In a further advantageous embodiment, an azimuthal section d(r=const.,θ)of the thickness profile d(r,θ) at a constant distance r from theelement axis is a linear function of the azimuth angle θ with the sameslope m but, in the ideal case, with two discontinuities at the azimuthangles θ=0 and θ=180°, respectively. At each discontinuity location,there is an abrupt step in the thickness by an amount of 180°/α. The twoabrupt steps at the discontinuity locations can also be distributed overan azimuth angle range of a few degrees. However, this has the result ofa non-optimized polarization distribution in the transition range.

In a further advantageous embodiment, an azimuthal section d(r=const.,θ)of the thickness profile d(r,θ) at a constant distance r from theelement axis and in a first azimuth angle range of 10°<θ<170° is alinear function of the azimuth angle θ with a first slope m, while in asecond azimuth angle range of 190°<θ<350°, the azimuthal section is alinear function of the azimuth angle θ with a second slope n. The slopesm and n have the same absolute magnitude but opposite signs. Themagnitude of the slopes m and n at a distance r from the element axis is

$|m| = {|n| = {\frac{180{^\circ}}{{\alpha\pi}\; r}.}}$

With this arrangement, the thickness profile for all azimuth angles,including θ=0 and θ=180°, is a continuous function without abruptchanges in thickness.

In a further advantageous embodiment, the polarization-modulatingoptical element is divided into a large number of planar-parallelportions of different thickness or comprises at least twoplanar-parallel portions. These portions can for example be configuredas sectors of a circle, but they could also have a hexagonal, square,rectangular, or trapezoidal shape.

In a further advantageous embodiment, a pair of first plan-parallelportions are arranged on opposite sides of a central element axis ofsaid polarization-modulating optical element, and a pair of secondplan-parallel portions are arranged on opposite sides of said elementaxis and circumferentially displaced around said element axis withrespect to said first plan-parallel portions, wherein each of said firstportions has a thickness being different from a thickness of each ofsaid second portions.

In a further advantageous embodiment, a plane of oscillation of linearlypolarized light passing through the polarization-modulating opticalelement is rotated by a first angle of rotation β₁ within at least oneof said first plan-parallel portions and by a second angle of rotationβ₂ within at least one of said second plan-parallel portions, such thatβ₁ and β₂ are approximately conforming or conform to the expression|β₂−β₁|=(2n+1)·90°, with n representing an integer.

In an advantageous embodiment, β₁ and β₂ are approximately conforming orconform to the expressions β₁=90°+p·180°, with p representing aninteger, and β₂=q·180°, with q representing an integer other than zero.As will discussed below in more detail, such an embodiment of thepolarization modulating optical element may be advantageously used inaffecting the polarization of traversing polarized light such thatexiting light has a polarization distribution being—depending of theincoming light—either approximately tangentially or approximatelyradially polarized.

The pair of second plan-parallel portions may particularly becircumferentially displaced around said element axis with respect tosaid pair of first plan-parallel portions by approximately 90°.

In a further advantageous embodiment, said pair of first plan-parallelportions and said pair of second plan-parallel portions are arranged onopposite sides of a central opening or a central obscuration of saidpolarization-modulating optical element.

Adjacent portions of said first and second pairs can be spaced apartfrom each other by regions being opaque to linearly polarized lightentering said polarization-modulating optical element. Said portions ofsaid first and second group can particularly be held together by amounting. Said mounting can be opaque to linearly polarized lightentering said polarization-modulating optical element. The mounting canhave a substantially spoke-wheel shape.

In a further advantageous embodiment, the polarization-modulatingoptical element comprises a first group of substantially planar-parallelportions wherein a plane of oscillation of traversing linearly polarizedlight is rotated by a first angle of rotation β₁, and a second group ofsubstantially planar-parallel portions wherein a plane of oscillation oftraversing linearly polarized light is rotated by a second angle ofrotation, such that β₁ and β₂ are approximately conforming or conform tothe expression |β₂−β₁|=(2n+1)·90°, with n representing an integer.

In a further advantageous embodiment, β₁ and β₂ are approximatelyconforming to the expressions β₁=90°+p·180°, with p representing aninteger, and β₂=q·180°, with q representing an integer other than zero.

In a further advantageous embodiment, the thickness profile of thepolarization-modulating optical element has a continuous surface contourwithout abrupt changes in thickness, whereby an arbitrarily selectedpolarization distribution can be generated whose thickness profile isrepresented by a continuous function of the location.

To ensure an adequate mechanical stability of the optical element, it ispreferred to make the minimal thickness d_(min) of thepolarization-modulating optical element at least equal to 0.002 timesthe element diameter D.

If the optically active material used for the optical element also hasbirefringent properties as is the case for example with crystallinequartz, the birefringence has to be taken into account for light rayswhose direction of propagation deviates from the direction of theoptical crystal axis. A travel distance of 90°/a inside the crystalcauses a linear polarization to be rotated by 90°. If birefringence ispresent in addition to the rotating effect, the 90° rotation will beequivalent to an exchange between the fast and slow axis in relation tothe electrical field vector of the light. Thus, a total compensation ofthe birefringence is provided for light rays with small angles ofincidence if the distance travelled inside the crystal equals an integermultiple of 180°/α. In order to meet the aforementioned requirement formechanical stability while simultaneously minimizing the effects ofbirefringence, it is of advantage if the polarization-modulating opticalelement is designed with a minimum thickness of

${d_{\min} = {N \cdot \frac{90{^\circ}}{\alpha}}},$

where N represents a positive integer.

From a manufacturing point of view, it is advantageous to provide theoptical element with a hole at the center or with a central obscuration.

For light rays propagating not exactly parallel to the optical crystalaxis, there will be deviations of the angle of rotation. In addition,the birefringence phenomenon will have an effect. It is thereforeparticularly advantageous if the maximum angle of incidence of anincident light bundle with a large number of light rays within a spreadof angles relative to the optical crystal axis is no larger than 100mrad, preferably no larger than 70 mrad, and with special preference nolarger than 45 mrad.

In order to provide an even more flexible control over a state ofpolarization, an optical arrangement is advantageously equipped with adevice that allows at least one further polarization-modulating opticalelement to be placed in the light path. This furtherpolarization-modulating optical element can be an additional elementwith the features described above. However, it could also be configuredas a planar-parallel plate of an optically active material or anarrangement of two half-wavelength plates whose respective fast and slowaxes of birefringence are rotated by 45° relative to each other.

The further polarization-modulating optical element that can be placedin the optical arrangement can in particular be designed in such a waythat it rotates the oscillation plane of a linearly polarized light rayby 90°. This is particularly advantageous if the firstpolarization-modulating element in the optical arrangement produces atangential polarization. By inserting the 90°-rotator, the tangentialpolarization can be converted to a radial polarization.

In a further embodiment of the optical arrangement, it can beadvantageous to configure the further polarization-modulating opticalelement as a planar-parallel plate which works as a half-wavelengthplate for a half-space that corresponds to an azimuth-angle range of180°. This configuration is of particular interest if the firstpolarization-modulating optical element has a thickness profile(r=const.,θ) that varies only with the azimuth angle θ and if, in afirst azimuth angle range of 10°<θ<170°, the thickness profile(r=const.,θ) is a linear function of the azimuth angle θ with a firstslope m, while in a second azimuth angle range of 190°<θ<350°, thethickness profile is a linear function of the azimuth angle θ with asecond slope n, with the slopes m and n having the same absolutemagnitude but opposite signs.

The refraction occurring in particular at sloped surfaces of apolarization-modulating element can cause a deviation in the directionof an originally axis-parallel light ray after it has passed through thepolarization-modulating element. In order to compensate this type ofdeviation of the wave front which is caused by thepolarization-modulating element, it is advantageous to arrange acompensation plate in the light path of an optical system, with athickness profile of the compensation plate designed so that itsubstantially compensates an angular deviation of the transmittedradiation that is caused by the polarization-modulating optical element.Alternatively, an immersion fluid covering the profiled surface of thepolarization-modulating element could be used for the same purpose.

Principally, in order to achieve the effect of compensating for thedeviation in the direction of an originally axis-parallel light ray dueto the polarization-modulating element, it would be possible to use anon-birefringent material such as CaF₂ or fused silica as raw materialfor the compensation plate. However, significant drawbacks of such anoptical arrangement are as follows: CaF₂ is relatively difficult tohandle during the manufacturing of the compensation plate, which usuallymakes it necessary to enhance its thickness e.g. up to 5 mm to achievesufficient mechanical stability, leading to an enhancement of the spaceneeded in the optical design. Fused silica is relatively sensitive tothermal compaction leading to local variations of the density andnon-deterministic birefringence properties, which inadvertently modifiesor destroys the polarization distribution after the optical arrangement.Furthermore, since the refraction indices of CaF₂ or fused silica, onthe one hand, and the optically active material of thepolarization-modulating element, on the other hand, are not the same,the slopes in the thickness profiles of the compensation plate and thepolarization-modulating element (or the “wedge angles”) in theseelements, have to be different. With other words, the distance betweenthe curved surfaces of these elements is not constant, which leads to anon-symmetric ray displacement for a light ray passing through thearrangement.

In order to avoid the above drawbacks while achieving a compensation forthe deviation in the direction of an originally axis-parallel light dueto the polarization-modulating element, an optical arrangement accordingto a further aspect of the present invention comprises

a polarization-modulating optical element having a first thicknessprofile and comprising a first optically active material with a firstoptical axis, wherein said first thickness profile, as measured in thedirection of said optical axis, is variable,

and a compensation plate being arranged in the light path of the opticalsystem and having a second thickness profile configured to substantiallycompensate for angle deviations of transmitted radiation which arecaused by said polarization-modulating optical element,

wherein said compensation plate comprises a second optically activematerial with a specific rotation of opposite sign compared to saidfirst optically active material.

Like in the embodiments of the polarization-modulating element discussedabove, the first and second optically active materials could be solid orliquid optically active materials.

In an advantageous embodiment, the polarization-modulating opticalelement and the compensation plate are made of optical isomers. In aparticular advantageous embodiment, the polarization-modulating opticalelement and the compensation plate are made of optically activecrystalline quartz with clockwise and counterclockwise specificrotation. With other words, if the polarization-modulating opticalelement is made of R-quartz, the compensation plate is preferably madeof L-quartz and vice versa. In such a combination of optically activematerials with a specific rotation of opposite sign, a net change in thepolarization direction of any linear polarized light ray will stilloccur, but is now depending on the value of the difference in therespective thicknesses being passed in these optically active materials.Such an embodiment has, in particular, the following advantageouseffects:

-   a) Since the refractive indices in the R-quartz and the L-quartz are    substantially the same, the slopes in the thickness profiles of the    compensation plate and the polarization-modulating element (or the    “wedge angles” in these elements) may also be the same. In    particular, both elements may be in direct contact to each other    with their respective inclined or curved surface, in order to    effectively forming a common or single optical element having the    shape of a substantially plan-parallel plate. Alternatively, the    compensation plate and the polarization-modulating element may also    be arranged spaced apart from each other such that the distance    between the inclined or curved surfaces of these elements is    constant. As a consequence, any ray displacement that occurs for a    light ray passing through the arrangement of the compensation plate    and the polarization-modulating element will be symmetric.-   b) Any parts of the polarization-modulating element and the    corresponding counter-part of the compensation plate can be    substantially identical in geometry, making it possible to    identically perform the corresponding manufacturing process (i.e.    with the same programming of the tools used for the manufacturing    procedure).-   c) Since the polarization-modulating optical element and the    compensation plate are turning the direction of polarization of    linear polarized light into opposite directions, temperature-induced    modifications of the effective rotation of polarization in these    elements will be at least partially compensated. In particular, any    offset thickness of the compensation plate or the    polarization-modulating element, respectively, will be without    influence with regard to temperature changes, since the accompanying    temperature-induced modifications of the effective rotation in one    of the elements will be compensated by the opposed effective    rotation in the other element.-   d) Since both the polarization-modulating optical element and the    compensation plate are providing optical activity with a variable    thickness profile, the respective slopes in the thickness profiles    of the compensation plate and the polarization-modulating element    may be reduced for each of these elements, if compared to the case    where only the polarization-modulating optical element is made of    optically active material. In particular, in the specific case where    both the compensation plate and the polarization-modulating element    have a substantially wedge-shaped cross-section, a tangential    polarization distribution can be achieved with substantially half of    the slope of the inclined surface of the compensation plate or the    polarization-modulating element, respectively, if compared to the    case where only the polarization-modulating optical element is made    of optically active material. As a consequence of these reduced    slopes, the space needed in the optical design is reduced and the    manufacturing process is simplified due to less effort in the    abrasive treatment of the respective raw materials used for making    the compensation plate and the polarization-modulating element.

Polarization-modulating elements of the foregoing description, andoptical arrangements equipped with them, are advantageously used inprojection systems for microlithography applications. In particular,polarization-modulating elements of this kind and optical arrangementsequipped with them are well suited for projection systems in which theaforementioned immersion technique is used, i.e., where an immersionmedium with a refractive index different from air is present in thespace between the optical element nearest to the substrate and thesubstrate.

According to a further aspect of the present invention, apolarization-modulating optical element is provided,

wherein a plane of oscillation of a first linearly polarized light rayand a plane of oscillation of a second linearly polarized light ray arerotated, respectively, by a first angle of rotation and a second angleof rotation in such a way that said first angle of rotation is differentfrom said second angle of rotation;

wherein said polarization-modulating optical element comprises at leasttwo planar-parallel portions that consist of an optically activematerial; and

wherein said planar-parallel portions are arranged on a support platethat consists of an optically active material.

As a consequence of making the support plate—like the at least twoplanar-parallel portions which are arranged thereon—of an opticallyactive material, an enhanced durability of a wringing-connection betweenthe support plate and said planar-parallel portions can be achieved, inparticular under varying temperature conditions. The enhanced stabilityor durability of the wringing-connection particularly results from thefact that the support plate may be made from the same material and evenwith the same crystal orientation. Physical properties as the refractionnumbers or expansion coefficients of the optically quartz material inthe support plate and the plan-parallel portions can be made rotationalsymmetric with regard to the respective optical crystal axis. Inparticular, it can be achieved that the thermal expansion coefficientsof the support plate and the plan-parallel portions (e.g. sector-shapedparts) are identical. Therefore, if the temperature changes in thedirect environment of the contact region between the support plate andthe planar-parallel portions (e.g. due to laser irradiation during themicrolithography process or during applying antireflection coatings),the temperature increase and the thermal expansion are the same in thesupport plate and the planar-parallel portions, so that the risk of atemperature- or stress-induced loose of contact between these element issignificantly reduced or eliminated, if e.g. compared to the use of asupport plate made of CaF₂ or fused silica (SiO₂). A further advantageis that the use of an optical active material such as crystalline quartzavoids compaction effects as occurring e.g. in fused silica.

According to a further aspect the present invention relates to aprojection system, comprising a radiation source, an illumination systemoperable to illuminate a structured mask, and a projection objective forprojecting an image of the mask structure onto a light-sensitivesubstrate,

wherein said projection system comprises an optical system comprising anoptical axis or a preferred direction given by the direction of a lightbeam propagating through the optical system,

the optical system comprising a temperature compensatedpolarization-modulating optical element described by coordinates of acoordinate system, wherein one preferred coordinate of the coordinatesystem is parallel to the optical axis or parallel to said preferreddirection;

said temperature compensated polarization-modulating optical elementcomprising a first and a second polarization-modulating optical element,the first and/or the second polarization-modulating optical elementcomprising solid and/or liquid optically active material and a profileof effective optical thickness, wherein the effective optical thicknessvaries at least as a function of one coordinate different from thepreferred coordinate of the coordinate system, in addition oralternative the first and/or the second polarization-modulating opticalelement comprises solid and/or liquid optically active material, whereinthe effective optical thickness is constant as a function of at leastone coordinate different from the preferred coordinate of the coordinatesystem;

wherein the first polarization-modulating optical element comprisesoptically active material with a specific rotation of opposite signcompared to the optically active material of the secondpolarization-modulating optical element.

Due to the presence of two polarization-modulating optical elementscomprising optically active materials having specific rotations ofopposite sign, the temperature effects in both these elements at leastpartially compensate each other, so that the combined system of thesetwo elements has a reduced temperature dependence regarding the changeof the polarization. Consequently, even under conditions of temperaturevariation, a change of the polarization state of light passing bothelements is reduced, or the polarization state even remains unchanged.As a consequence of said compensation effect, a detrimental effect oftemperature variations on the polarization state of light passingthrough said system can be reduced or avoided even with relatively largethicknesses (e.g. of several mm) of said elements.

The above compensation concept may particularly be used in a system oftwo plane plates with a first and a second thickness in the direction ofthe propagating light beam, said plates being made of optically activequartz with clockwise and counterclockwise specific rotation.Furthermore, according to the present invention said plane plates mayhave either substantially the same thickness or different thicknesses.In the case of substantial identical thicknesses of the two planeplates, a resulting effect of the whole arrangement of the two platesmay be avoided, so that such arrangement is particularly suited e.g. asa polarization-neutral support of a micro-optical element such as a DOE.In the case of different thicknesses of the two plane plates, aresulting effect of the whole arrangement of the two plates can beachieved to provide a polarization-modulating arrangement having, due tothe partial compensation effect mentioned before, a relatively weaksensitivity to temperature variations even if relatively largethicknesses for the plane plates are used in view of manufacturingaspects.

Therefore, according to a further aspect the present invention relatesto an optical system comprising an optical axis or a preferred directiongiven by the direction of a light beam propagating through the opticalsystem,

the optical system comprising a temperature compensatedpolarization-modulating optical element described by coordinates of acoordinate system, wherein one preferred coordinate of the coordinatesystem is parallel to the optical axis or parallel to said preferreddirection;

said temperature compensated polarization-modulating optical elementcomprising a first and a second polarization-modulating optical element,the first and/or the second polarization-modulating optical elementcomprising solid and/or liquid optically active material and a profileof effective optical thickness, wherein the effective optical thicknessvaries at least as a function of one coordinate different from thepreferred coordinate of the coordinate system, in addition oralternative the first and/or the second polarization-modulating opticalelement comprises solid and/or liquid optically active material, whereinthe effective optical thickness is constant as a function of at leastone coordinate different from the preferred coordinate of the coordinatesystem; wherein the first polarization-modulating optical elementcomprises optically active material with a specific rotation of oppositesign compared to the optically active material of the secondpolarization-modulating optical element;

wherein said first and second polarization-modulating optical elementsare plane plates with a first and a second thickness in the direction ofthe propagating light beam, said plates being made of optically activequartz with clockwise and counterclockwise specific rotation; whereinthe first and the second thickness are different from each other.

Preferably, the absolute value of the difference of the first and thesecond thickness is smaller than the thickness of the smaller plate.

According to a further aspect the present invention relates to anoptical system having an optical axis, the optical system comprising afirst plane plate with a first thickness in the direction of the opticalaxis and a second plane plate with a second thickness in the directionof the optical axis, wherein said first and second plane plates are madeof optically active quartz with specific rotations opposite to eachother; and wherein the first and the second thickness are different fromeach other.

According to a further aspect the present invention relates to aprojection system comprising a radiation source, an illumination systemoperable to illuminate a structured mask, and a projection objective forprojecting an image of the mask structure onto a light-sensitivesubstrate, wherein said projection system comprises an optical systemhaving an optical axis, the optical system comprising a first planeplate with a first thickness in the direction of the optical axis and asecond plane plate with a second thickness in the direction of theoptical axis; wherein said first and second plane plates are made ofoptically active quartz with specific rotations opposite to each other.

Further advantageous aspects can be gathered from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereinafter be explained in more detail withreference to the attached drawings, wherein:

FIG. 1 illustrates a polarization-modulating optical element with athickness profile;

FIG. 2 schematically illustrates how the plane of oscillation is rotatedwhen a linearly polarized light ray propagates along the optical axis inan optically active crystal;

FIG. 3 illustrates a first exemplary embodiment of apolarization-modulating optical element;

FIG. 4 a schematically illustrates a second exemplary embodiment of apolarization-modulating optical element;

FIG. 4 b illustrates the thickness profile as a function of the azimuthangle in the embodiment of the polarization-modulating optical elementof FIG. 4 a;

FIG. 4 c illustrates the thickness profile as a function of the azimuthangle in a further embodiment of the polarization-modulating opticalelement;

FIG. 4 d illustrates the thickness profile as a function of the azimuthangle in the embodiment of the polarization-modulating optical elementof FIG. 3;

FIG. 4 e illustrates the thickness profile as a function of the azimuthangle in a further embodiment of the polarization-modulating opticalelement;

FIG. 4 f schematically illustrates a further exemplary embodiment of apolarization-modulating optical element;

FIG. 4 g-h schematically illustrate a further exemplary embodiment of apolarization-modulating optical element in a plan view (FIG. 4 g) and afront-side view (FIG. 4 h);

FIG. 5 schematically illustrates the polarization distribution of abundle of light rays before and after passing through thepolarization-modulating optical element with the thickness profileaccording to FIG. 3 or 4 d;

FIG. 6 schematically illustrates the polarization distribution of abundle of light rays before and after passing through an opticalarrangement with the polarization-modulating optical element with thethickness profile according to FIG. 3 and a furtherpolarization-modulating optical element;

FIG. 7 a schematically illustrates the polarization distribution of abundle of light rays before and after passing through an opticalarrangement with the polarization-modulating optical element with thethickness profile according to FIG. 4 e and a planar-parallel plate, onehalf of which is configured as a half-wave plate;

FIG. 7 b shows a plan view of a planar-parallel plate, one half of whichis configured as a half-wave plate;

FIG. 8 schematically illustrates a microlithography projection systemwith a polarization-modulating optical element;

FIG. 9 schematically shows a parallel plane plate of optical activematerial used as a polarization-modulating element by adjusting itstemperature and/or temperature profile;

FIG. 10 shows a combination of a parallel plate of optical activematerial with a plate made of birefringent material;

FIG. 11 shows schematically a temperature compensatedpolarization-modulating optical element for the application in anoptical system;

FIG. 12-13 show schematically temperature compensated optical elementsaccording to further embodiments of the present invention;

FIG. 14 a-b are schematic illustrations to demonstrate the effect of atemperature compensated optical element according to a furtherembodiment of the present invention;

FIG. 15-16 are schematic illustrations of a temperature compensated greyfilter (FIG. 15) and a temperature compensated diffractive opticalelement (FIG. 16) according to further embodiments of the presentinvention;

FIGS. 17 a-d show different views on a combination of apolarization-modulating optical element with a compensation plate in apreferred embodiment of the present invention; and

FIGS. 18 a-c show thickness profiles as a function of the azimuth anglefor different combinations of a polarization-modulating optical elementwith a compensation plate.

DETAILED DESCRIPTION

FIG. 1 illustrates a polarization-modulating optical element 1 of anoptically active material. Particularly well suited for this purpose areoptically active crystals with at least one optical crystal axis whichare transparent for the wavelength of the light being used. For exampleTeO₂ works in a range of wavelengths from 1000 nm down to 300 nm, AgGaS₂works from 500 nm to 480 nm, and quartz from 800 nm down to 193 nm. Thepolarization-modulating optical element 1 is designed so that theelement axis is oriented parallel to the optical crystal axis. In orderto produce a selected polarization distribution, the optical element 1is designed with a thickness profile (measured parallel to the elementaxis EA) which varies in the directions perpendicular to the elementaxis EA, also comprising variations in thickness of the optical elementin an azimuth direction θ (see FIG. 3) at e.g. a fixed distance of theelement axis EA.

FIG. 2 will serve to explain the function of optically active crystals,and in particular of polarization-modulating elements made from suchcrystals, in more detail.

Optically active crystals have at least one optical axis OA which isinherent in the crystal structure. When linearly polarized light travelsalong this optical axis OA, the plane of oscillation of the electricalfield vector 206 is rotated by an angle β of proportionate magnitude asthe distance d travelled by the light inside the crystal 202. Theproportionality factor between distance d and angle of rotation is thespecific rotation α. The latter is a material-specific quantity and isdependent on the wavelength of the light rays propagating through thecrystal. For example in quartz, the specific rotation at a wavelength of180 nm was measured as about α=(325.2±0.5)°/mm, at 193 nm α=323.1°/mm ata temperature of 21.6° C.

It is also important for the present invention, applying opticallyactive materials in an illumination system and/or an objective of aprojection optical system of e.g. a projection apparatus used inmicrolithography, that also the temperature dependency of the specificrotation is considered. The temperature dependency of the specificrotation α for a given wavelength is to a good and first linearapproximation given by α(T)=α₀(T₀)+γ*(T−T₀), where γ is the lineartemperature coefficient of the specific rotation α. In this case α(T) isthe optical activity coefficient or specific rotation at the temperatureT and α₀ is the specific rotation at a reference temperature T₀. Foroptical active quartz material the value γ at a wavelength of 193 nm andat room temperature is γ=2.36 mrad/(mm*K).

Referring again to FIG. 2, in particular, light that propagates insidethe crystal 202 along the optical axis OA is not subject to a linearbirefringence. Thus, when a linearly polarized light ray traverses anoptically active crystal 202 along the optical axis OA, its state ofpolarization remains the same except for the change in the spatialorientation of the plane of oscillation of the electrical field vector206 which depends on the distance d travelled by the light ray insidethe crystal 202.

Based on this property of an optically active crystal, it is possible toproduce an arbitrarily selected linear polarization distribution bydesigning the polarization-modulating optical element 1 of FIG. 1 with athickness profile that varies dependent on the location. The thicknessprofile is designed to have the effect that the directions ofpolarization of parallel linearly polarized light rays are rotated by anangle that varies dependent on the location where the light raytraverses the optical element.

More general, alternative or in addition to the variation of thethickness d=d(x,y) of the polarization-modulating element, the specificrotation α may itself be dependent on the location within the modulatingelement such that a becomes an α(x,y,z) or α(r,θ,z), where x,y or r,θare Cartesian or polar coordinates in a plane perpendicular to theelement axis EA (or alternative to the optical axis OA) of thepolarization-modulating element, as shown e.g. in FIG. 1, where z is theaxis along the element axis EA. Of course also a description inspherical-coordinates like r,θ,φ, or others is possible. Taking intoaccount the variation of the specific rotation α, thepolarization-modulating optical element in general comprises a varyingprofile of the “optical effective thickness D” defined asD(x,y)=d(x,y)*α(x,y), if there is no dependency of α in z-direction. Inthe case that α may also depend on the z-direction (along the opticalaxis or element axis EA, or more general along a preferred direction inan optical system or a direction parallel to the optical axis of anoptical system) D has to be calculated by integration D(x,y)=∫α(x,y,z)dz(x,y), along the polarization-modulating optical element. In general,if a polarization-modulating optical element is used in an opticalsystem, having an optical axis or a preferred direction defined by thepropagation of a light beam through the optical system, the opticaleffective thickness D is calculated by integrating the specific rotationa along the light path of a light ray within the polarization-modulatingoptical element. Under this general aspect the present invention relatesto an optical system comprising an optical axis or a preferred directiongiven by the direction of a light beam propagating through the opticalsystem. The optical system also comprises a polarization-modulatingoptical element described by coordinates of a coordinate system, whereinone preferred coordinate of the coordinate system is parallel to theoptical axis of the optical system or parallel to the preferreddirection. As an example, in the above case this preferred direction wasthe z-coordinate which is the preferred coordinate. Additionally thepolarization-modulating optical element comprises optical activematerial and also a profile of effective optical thickness D as definedabove, wherein the effective optical thickness D varies at least as afunction of one coordinate different from the preferred coordinate ofthe coordinate system describing the polarization-modulating opticalelement. In the above example the effective optical thickness D variesat least as a function of the x- or y-coordinate, different from thez-coordinate (the preferred coordinate). There are different independentmethods to vary the effective optical thickness of an optical activematerial. One is to vary the specific rotation by a selection ofappropriate materials, or by subjecting the optically active material toa non-uniform temperature distribution, or by varying the geometricalthickness of the optically active material. Also combinations of thementioned independent methods result in a variation of the effectiveoptical thickness of an optical active material.

FIG. 3 illustrates an embodiment of the polarization-modulating opticalelement 301 which is suited specifically for producing a tangentialpolarization. A detailed description will be presented in the context ofFIGS. 4 d and 5. The embodiment illustrated in FIG. 3 will serve tointroduce several technical terms that will be used hereinafter with thespecific meanings defined here.

The polarization-modulating optical element 301 has a cylindrical shapewith a base surface 303 and an opposite surface 305. The base surface303 is designed as a planar circular surface. The element axis EAextends perpendicular to the planar surface. The opposite surface 305has a contour shape in relation to the element axis EA in accordancewith a given thickness profile. The optical axis of the optically activecrystal runs parallel to the element axis EA. The reference axis RA,which extends in the base plane, intersects the element axis at a rightangle and serves as the reference from which the azimuth angle θ ismeasured. In the special configuration illustrated in FIG. 3, thethickness of the polarization-modulating optical element 301 is constantalong a radius R that is perpendicular to the element axis EA anddirected at an angle θ relative to the reference axis RA. Thus, thethickness profile in the illustrated embodiment of FIG. 3 depends onlyon the azimuth angle θ and is given by d=d (θ). The optical element 301has an optional central bore 307 coaxial with the element axis EA. Inanother preferred embodiment of the polarization-optical element thethickness may vary along the radius R such that the thickness profile isd=d(R,θ). In a further more generalized preferred embodiment thethickness profile shown in FIG. 3 is not representing the geometricalthickness d of the polarization-optical element, as described above, butthe profile represents the optical effective thickness D=D(R,θ)=D(x,y),depending on the used coordinate system. In this case also any profileof the specific rotation like e.g. α=α(x,y)=α(R,θ) orα=α(x,y,z)=α(R,θ,z) is considered in the profile of thepolarization-modulating optical element which is effective for a changein the direction of the polarization plane of a passed light beam.

In addition it should be mentioned that the polarization-modulatingoptical element 301 not necessary need to comprise a planar base surface303. This surface in general can also comprise a contour shaped surfacee.g. similar or equal to the surface as designated by 305 shown in FIG.3. In such a case it is of advantage to describe the contour surfaces303 and 305 relative to a plane surface perpendicular to the opticalaxis or element axis.

FIG. 4 a schematically illustrates a further embodiment of thepolarization-modulating optical element 401. The element axis EA throughthe center of the polarization-modulating optical element 401 in thisrepresentation runs perpendicular to the plane of the drawing, and theoptical crystal axis of the crystal runs parallel to the element axis.Like the embodiment of FIG. 3, the polarization-modulating opticalelement 401 has an optional central bore 407. Thepolarization-modulating optical element 401 is divided into a largenumber of planar-parallel portions 409 in the shape of sectors of acircle which differ in their respective thicknesses. Alternativeembodiments with different shapes of the portions 409 are conceivable.They could be configured, e.g., as hexagonal, square, rectangular ortrapeze-shaped raster elements.

As described in connection with FIG. 3, the embodiment according to FIG.4 a can be modified such that the different thicknesses of the sectorsshould be understood as different effective optical thicknesses D. Inthis case the specific rotation a may vary from one segment to the othertoo. To manufacture such an embodiment, the polarization-modulatingoptical element can e.g. have a shape as shown in FIG. 4 a in which thesectors 409 are at least partly exchanged e.g. by any optical inactivematerial, which is the simplest case to vary the specific rotation α tozero. Also as a further embodiment the sectors 409 may be replaced bycuvettes or cells which are filed with an optical active or opticalinactive liquid. In this case the polarization-modulating opticalelement may comprise optical active and optical inactive sections. Ifthe sectors 409 are only party replaced by cuvettes or if at least onecuvette is used in the polarization-modulating optical element 401, acombination of e.g. optical active crystals with e.g. optical active oroptical inactive liquids in one element 40 is possible. Such an opticalsystem according to the present invention may comprise apolarization-modulating optical element which comprises an opticallyactive or an optically inactive liquid and/or an optically activecrystal. Further, it is advantageously possible that thepolarization-modulating optical element of the optical system accordingto the present invention comprises clockwise and counterclockwiseoptically active materials. These materials could be solid or liquidoptically active materials. Using liquids in cuvettes has the advantagethat by changing the liquids, or the concentration of the optical activematerial within the liquid, the magnitude of the change in polarizationcan be easily controlled. Also any thermal changes of the specificrotation a due to the thermal coefficient γ of the specific rotation acan be controlled e.g. by temperature control of the optical activeliquid such that either the temperature is constant within the cuvette,or that the temperate has predefined value T such that the specificrotation will have the value α(T)=α₀(T₀)+γ*(T−T₀). Also the formation ofa certain temperature distribution within the liquid may be possiblewith appropriate heating and/or cooling means controlled by controlmeans.

The optical systems in accordance with the present inventionadvantageously modify respective planes of oscillation of a firstlinearly polarized light ray and a second linearly polarized light ray.Both light rays propagating through the optical system, and being atleast a part of the light beam propagating through the optical system.The light rays are also passing the polarization-modulating opticalelement with different paths, and are rotated by a respective first andsecond angle of rotation such that the first angle is different of thesecond angle. In general the polarization-modulating optical element ofthe optical systems according to the present invention transforms alight bundle with a first linear polarization distribution, which enterssaid polarization-modulating optical element, into a light bundleexiting said polarization-modulating optical element. The exiting lightbundle has a second linear polarization distribution, wherein the secondlinear polarization distribution is different from the first linearpolarization distribution.

FIG. 4 b shows the thickness profile along an azimuthal sectiond(r=const.,θ) for the polarization-modulating optical element 401divided into sectors as shown in FIG. 4 a. The term azimuthal section asused in the present context means a section traversing the thicknessprofile d(θ,r) along the circle 411 marked in FIG. 4 a, i.e., extendingover an azimuth angle range of 0°≦θ≦360° at a constant radius r. Ingeneral the profile shows the optical effective thickness D=D(θ) along acircle 411.

An azimuthal section of a polarization-modulating optical element 401that is divided into sector-shaped portions has a stair-shaped profilein which each step corresponds to the difference in thickness d oroptical effective thickness D between neighbouring sector elements. Theprofile has e.g. a maximum thickness d_(max) and a minimum thicknessd_(min). In order to cover a range of 0≦β≦360° for the range of theangle of rotation of the oscillation plane of linearly polarized light,there has to be a difference of 360°/α between d_(max) and d_(min).

The height of each individual step of the profile depends on the numbern of sector elements and has a magnitude of 360°/(n·α). At the azimuthangle θ=0°, the profile has a discontinuity where the thickness of thepolarization-modulating optical element 401 jumps from d_(min) tod_(max). A different embodiment of the optical element can have athickness profile in which an azimuthal section has two discontinuitiesof the thickness, for example at θ=0° and θ=180°.

In an alternative embodiment the profile has e.g. a maximum opticaleffective thickness D_(max) and a minimum optical effective thicknessD_(min), and the geometrical thickness d is e.g. constant, resulting ina variation of the specific rotation α of the individual segments 409 ofthe element 401. In order to cover a range of 0≦β≦360° for the range ofthe angle of rotation of the oscillation plane of linearly polarizedlight, there has to be a difference of 360°/d between α_(max) andα_(min). The change of the specific rotation of each individual step ofthe profile depends on the number n of sector elements 409 and has amagnitude of 360°/(n·d). At the azimuth angle θ=0°, the profile has adiscontinuity regarding the optical effective thickness where it jumpsfrom D_(min) to D_(max). It should be pointed out, that advantageouslyin this embodiment there is no discontinuity in the geometricalthickness d of the polarization-modulating element 401. Also thethickness profile of the optical effective thickness in which anazimuthal section has two discontinuities of the optical effectivethickness can easily be realized, for example at θ=0° and θ=180°. Torealize the defined changes in magnitude of the specific rotation ofΔα=360°/(n·d)(if there a n angular segments 409 to form the element401), the individual sector elements 409 are preferably made of orcomprises cuvettes or cells, filled with an optical active liquid withthe required specific rotation α. As an example, for the m-th sectorelement the specific rotation is α(m)=α_(min)+m*360°/(n·d), and 0≦m≦n.The required specific rotation e.g. can be adjusted by the concentrationof the optical active material of the liquid, or by changing the liquidmaterial itself.

In a further embodiment the segments 409 of a polarization-modulatingoptical element 401 may comprise components of solid optically activematerial (like crystalline quartz) and cells or cuvettes filled withoptically active material, and these components are placed behind eachother in the light propagation direction. Alternative or in addition thecuvette itself may comprise optically active material like crystallinequartz.

The polarization-modulating optical element of the foregoing descriptionconverts linearly polarized incident light into a linear polarizationdistribution in which the oscillation planes of linearly polarized lightrays are rotated by an angle that depends on the thickness (or opticaleffective thickness) of each individual sector element. However, theangle by which the direction of polarization is rotated is constant overan individual sector element. Thus, the distribution function for thedirections of the oscillation planes of the individual field vectorstakes only certain discrete values.

A continuous distribution of linear polarizations can be achieved withan optical element that has a continuously varying thickness (opticaleffective thickness) profile along an azimuthal section.

An example of a continuously varying thickness profile is illustrated inFIG. 4 c. The azimuthal section 411 in this embodiment shows a lineardecrease in thickness (in general optical effective thickness) with aslope m=−180°/(α·π) over an azimuth-angle range of 0≦θ≦360°. Here theslope is defined a slope of a screw. Alternatively the slope can bedefined by m=−180°/(α*π*r) where r is the radius of a circle centred atthe element axis EA. In this case the slope depends on the distance ofthe element axis, e.g. if the polarization-modulating optical element301 has a given constant screw-slope (lead of a screw).

The symbol a in this context stands for the specific rotation of theoptically active crystal. As in the previously described embodiment ofFIG. 4 b, the thickness profile of FIG. 4 c has likewise a discontinuityat the azimuth angle θ=0°, the thickness of the polarization-modulatingoptical element 401 jumps from d_(min) to d_(max) by an amount ofapproximately 360°/α.

A further embodiment of a polarization-modulating optical element whichis shown in FIG. 4 d has a thickness profile (in general opticaleffective thickness profile) which is likewise suitable for producing acontinuous distribution of linear polarizations, in particular atangentially oriented polarization. This thickness profile correspondsto the embodiment shown in FIG. 3, in which the angle θ is measured incounterclockwise direction. The azimuthal section 411 in this embodimentis a linear function of the azimuth angle θ with a slope m=−180°/(α·π)over each of two ranges of 0<θ<180° and 180°<θ<360°. The thicknessprofile has discontinuities at θ=0° and θ=180° where the thickness risesabruptly from d_(min) to d_(max) by an amount of 180°/α.

FIG. 4 e represents the thickness profile (in general optical effectivethickness profile) along an azimuthal section for a further embodimentof the polarization-modulating optical element 401. The azimuthalsection is in this case a linear function of the azimuth angle θ with afirst slope m for 0<θ<180° and with a second slope n for 180°<θ<360°.The slopes m and n are of equal absolute magnitude but have oppositesigns. The respective amounts for m and n at a distance r from theelement axis are m=−180°/(α·π·r) and n=180°/(α·π·r). While thedifference between the minimum thickness d_(min) and the maximumthickness d_(max) is again approximately 180°/α, i.e., the same as inthe embodiment of FIG. 4 d, the concept of using opposite signs for theslope in the two azimuth angle ranges avoids the occurrence ofdiscontinuities.

Additionally it is mentioned that for certain special applicationsclockwise and counterclockwise optically active materials are combinedin a polarization-modulating optical element.

As the slope of the thickness profile along an azimuthal sectionincreases strongly with smaller radii, it is advantageous from amanufacturing point of view to provide a central opening 407 or acentral obscuration in a central portion around the central axis of thecircular polarization-modulating optical element.

It is furthermore advantageous for reasons of mechanical stability todesign the polarization-modulating optical element with a minimumthickness d_(min) of no less than two thousandths of the elementdiameter. It is particularly advantageous to use a minimum thickness ofd_(min)=N·90°/α, where N is a positive integer. This design choiceserves to minimize the effect of birefringence for rays of an incidentlight bundle which traverse the polarization-modulating element at anangle relative to the optical axis.

FIG. 4 f schematically illustrates a further embodiment 421 of thepolarization-modulating optical element. As in FIG. 4 a, the elementaxis EA through the center of the polarization-modulating opticalelement 421 runs perpendicular to the plane of the drawing, and theoptical crystal axis runs parallel to the element axis. However, incontrast to the embodiments of FIGS. 3 and 4 a where thepolarization-modulating optical elements 301, 401 are made preferably ofone piece like in the case of crystalline material like crystallinequartz, the polarization-modulating optical element 421 comprises offour separate sector-shaped parts 422, 423, 424, 425 of an opticallyactive crystal material which are held together by a mounting device 426which can be made, e.g., of metal and whose shape can be described as acircular plate 427 with four radial spokes 428. The mounting ispreferably opaque to the radiation which is entering thepolarization-modulating optical element, thereby serving also as aspacer which separates the sector-shaped parts 422, 423, 424, 425 fromeach other. Of course the embodiment of the present invention accordingto FIG. 4 f is not intended to be limited to any specific shape and areaof mounting device 426, which may also be omitted.

According to an alternate embodiment not illustrated in FIG. 4 f,incident light which is entering the polarization-modulating opticalelement can also be selectively directed onto the sector-shaped parts,e.g. by means of a diffractive structure or other suitable opticalcomponents.

The sector-shaped parts 422 and 424 have a first thickness d1 which isselected so that the parts 422 and 424 cause the plane of oscillation oflinearly polarized axis-parallel light to be rotated by 90°+p·180°,where p represents an integer. The sector-shaped parts 423 and 425 havea second thickness d2 which is selected so that the parts 423 and 425cause the plane of oscillation of linearly polarized axis-parallel lightto be rotated by q·180°, where q represents an integer other than zero.Thus, when a bundle of axis-parallel light rays that are linearlypolarized in the y-direction enters the polarization-modulating opticalelement 421, the rays that pass through the sector-shaped parts 423 and425 will exit from the polarization-modulating optical element 421 withtheir plane of oscillation unchanged, while the rays that pass throughthe sector-shaped parts 422 and 424 will exit from thepolarization-modulating optical element 421 with their plane ofoscillation rotated into the x-direction. As a result of passing throughthe polarization-modulating optical element 421, the exiting light has apolarization distribution which is exactly tangential at the centrelines429 and 430 of the sector-shaped parts 422, 423, 424, 425 and whichapproximates a tangential polarization distribution for the rest of thepolarization-modulating optical element 421.

When a bundle of axis-parallel light rays that are linearly polarized inthe x-direction enters the polarization-modulating optical element 421,the rays that pass through the sector-shaped parts 423 and 425 will exitfrom the polarization-modulating optical element 421 with their plane ofoscillation unchanged, while the rays that pass through thesector-shaped parts 422 and 424 will exit from thepolarization-modulating optical element 421 with their plane ofoscillation rotated into the y-direction. As a result of passing throughthe polarization-modulating optical element 421, the exiting light has apolarization distribution which is exactly radial at the centrelines 429and 430 of the sector-shaped parts 422, 423, 424, 425 and whichapproximates a radial polarization distribution for the rest of thepolarization-modulating optical element 421.

Of course the embodiment of the present invention according to FIG. 4 fis not intended to be limited to the shapes and areas and the number ofsector-shaped parts exemplarily illustrated in FIG. 4 f, so that othersuitable shapes (having for example but not limited to trapeze-shaped,rectangular, square, hexagonal or circular geometries) as well as moreor less sector-shaped parts 422, 423, 424 and 425 can be used.Furthermore, the angles of rotation β₁ and β₂ provided by thesector-shaped parts 422, 423, 424, 425 (i.e. the correspondingthicknesses of the sector-shaped parts 422, 423, 424, 425) may be moregenerally selected to approximately conform to the expression|β₂−β₁|=(2n+1)·90°, with n representing an integer, for example toconsider also relative arrangements where incoming light is used havinga polarization plane which is not necessarily aligned with the x- ory-direction. With the embodiments as described in connection with FIG. 4f it is also possible to approximate polarization distributions with atangential polarization.

In order to produce a tangential polarization distribution from linearlypolarized light with a wave length of 193 nm and a uniform direction ofthe oscillation plane of the electric field vectors of the individuallight rays, one can use for example a polarization-modulating opticalelement of crystalline quartz with the design according to FIGS. 3 and 4d. The specific rotation a of quartz for light with a wavelength of 193nm is in the range of (325.2±0.5)°/mm, which was measured at awavelength of 180 nm, or more precise it is 321.1°/mm at 21.6° C. Thestrength and effect of the optical activity is approximately constantwithin a small range of angles of incidence up to 100 mrad. Anembodiment could for example be designed according to the followingdescription: An amount of 276.75 μm, which approximately equals 90°/α,is selected for the minimum thickness d_(min), if crystalline quartz isused. Alternatively, the minimum thickness d_(min) can also be aninteger multiple of this amount. The element diameter is 110 mm, withthe diameter of the optically active part being somewhat smaller, forexample 105 mm. The base surface is designed as a planar surface asillustrated in FIG. 3. The opposite surface has a thickness profiled(r,θ) in accordance with FIG. 4 d. The thickness profile is defined bythe following mathematical relationships:

$\begin{matrix}{{D( {r,\theta} )} = \begin{matrix}{276.75 + {\frac{{180{^\circ}} - \theta}{180{^\circ}} \cdot}} \\{553.51\mspace{14mu} {\mu m}}\end{matrix}} & {{{for}\mspace{14mu} 0} \leq \theta \leq {180{^\circ}\mspace{14mu} {and}\mspace{14mu} r} > {\frac{10.5}{2}{mm}}} \\{{D( {r,\theta} )} = \begin{matrix}{276.75 + {\frac{{360{^\circ}} - \theta}{180{^\circ}} \cdot}} \\{553.51\mspace{14mu} {\mu m}}\end{matrix}} & {{{for}\mspace{14mu} 180} \leq \theta \leq {360{^\circ}\mspace{14mu} {and}\mspace{14mu} r} > {\frac{10.5}{2}{mm}}} \\{{D( {r,\theta} )} = 0} & {{{for}\mspace{14mu} r} \leq {\frac{10.5}{2}{mm}}}\end{matrix}$

The above mentioned data are based exemplarily for a specific rotation αof (325.2±0.5)°/mm. If the specific rotation a changes to 321.1°/mm, thevalue at 193 nm and at a temperature of 21.6° C., the thickness profilewill change as follows:

$\begin{matrix}{{D( {r,\theta} )} = \begin{matrix}{278.6 + {\frac{{180{^\circ}} - \theta}{180{^\circ}} \cdot}} \\{557\mspace{14mu} {\mu m}}\end{matrix}} & {{{for}\mspace{14mu} 0} \leq \theta \leq {180{^\circ}\mspace{14mu} {and}\mspace{14mu} r} > {\frac{10.5}{2}{mm}}} \\{{D( {r,\theta} )} = \begin{matrix}{278.6 + {\frac{{360{^\circ}} - \theta}{180{^\circ}} \cdot}} \\{557\mspace{14mu} {\mu m}}\end{matrix}} & {{{for}\mspace{14mu} 180} \leq \theta \leq {360{^\circ}\mspace{14mu} {and}\mspace{14mu} r} > {\frac{10.5}{2}{mm}}} \\{{D( {r,\theta} )} = 0} & {{{for}\mspace{14mu} r} \leq {\frac{10.5}{2}{mm}}}\end{matrix}$

The polarization-modulating optical element according to this embodimenthas a central opening 407 with a diameter 10.5, i.e., one-tenth of themaximum aperture. The thickness maxima and minima, which are found atthe discontinuities, are 830.26 um and 276.75 um, respectively for thefirst given example.

The embodiment of the foregoing description can be produced with arobot-polishing process. It is particularly advantageous to produce thepolarization-modulating element from two wedge-shaped or helicallyshaped half-plates which are seamlessly joined together after polishing.If the element is produced by half-plates, it is easy and in someapplications of additional advantage to use one clockwise and onecounterclockwise optically active material like clockwise crystallineand counterclockwise crystalline quartz (R-quartz and L-quartz).

FIGS. 4 g and 4 h schematically show a further preferred embodiment of apolarization-modulating element 450 in a plan-view (FIG. 4 g) and afront-side view (FIG. 4 h). The polarization-modulating element 450 ofFIG. 4 g comprises a support plate 451 and two sector-shaped parts 452and 453 which are fixed, by means of wringing, on said support plate451.

The embodiment of FIG. 4 g,h is characterized in that the support plate451 is made of optically active crystalline quartz, with the opticalcrystal axis being perpendicular to the surface of the support plate451, i.e. parallel to the z-direction in the coordinate system alsoshown in FIG. 4 g,h. The direction of the optical crystal axis in thesupport plate 451 is also shown in FIG. 4 h and referenced with “oa-1”.Furthermore, and like in the embodiment described with reference to FIG.4 f, the sector-shaped parts 452 and 453 are also made of opticallyactive crystalline quartz, with the optical crystal axis beingperpendicular to their surface, i.e. also parallel to the z-direction.The direction of the optical crystal axis in the sector-shaped parts 452and 453 is also shown in FIG. 4 h and referenced with “oa-2”.

The thickness d₂ of the sector-shaped parts 452 and 453 is such that theorientation of polarization of linearly polarized light with normalincidence on the light entrance side of the polarization-modulatingelement 450 is rotated by an angle of 90°. The thickness d₁ of thesupport plate 451 is such that the orientation of polarization oflinearly polarized light with normal incidence on the light entranceside of the support plate 451 is rotated by an angle of q*180°, with qbeing an integer larger than zero. For use of synthetic, opticallyactive crystalline quartz having a specific rotation of 323.1°/mm for awavelength of 193 nm and a temperature of 21.6° C., this condition meansthat the thickness d of the support plate is d₁≅q*557 μm. As aconsequence, the support plate 451 behaves neutral with respect to theeffect of the polarization-modulating element 450 in the sense that thepolarization distribution of light entering the light entrance side ofthe support plate 451 with normal incidence is identical to thepolarization distribution of this light after having passed the supportplate 451, i.e. at the light exit side of the support plate 451.

A significant advantage of this embodiment using the above-describedsupport plate 451 made of optically active quartz is that an enhanceddurability of the wringing-connection between the support plate 451 andsector-shaped parts 452 and 453 can be achieved, in particular undervarying temperature conditions. The enhanced stability or durability ofthe wringing-connection is a result from the fact that the support plate451 and the sector-shaped parts 452 and 453 are not only made from thesame material, but also have, in addition to the identity of materials,the same crystal orientation. In the crystal orientation according tothe configuration shown in FIGS. 4 g and 4 h, physical properties as therefraction numbers or expansion coefficients of the optically activequartz material in the support plate 451 and the sector-shaped parts 452and 453 are rotational symmetric with regard to the respective opticalcrystal axis. In particular, the thermal expansion coefficients of thesupport plate 451 and the sector-shaped parts 452 and 453 are identical.Therefore, if the temperature changes in the direct environment of thecontact region between the support plate 451 and the sector-shaped parts452 and 453 (e.g. due to laser irradiation during the microlithographyprocess or during applying antireflection coatings), the temperatureincrease and the thermal expansion are the same in the support plate 451and the sector-shaped parts 452 and 453, so that the risk of atemperature- or stress-induced loose of contact between these element issignificantly reduced or eliminated, if e.g. compared to the use of asupport plate made of CaF₂ or fused silica.

In the embodiment illustrated in FIGS. 4 g and 4 h, the support plate451 and the sector-shaped parts 452 and 453 are all made of right-handedoptically active quartz. The invention is however not limited thereto.Alternatively, the support plate 451 and the sector-shaped parts 452 and453 can be made of left-handed optically active quartz. In a furtherpreferred embodiment, the support plate 451 is made of right-handedoptically active quartz, and the sector-shaped parts 452 and 453 aremade of left-handed optically active quartz, or vice versa. An advantageof the use of both R-quartz and L-quartz is that the sector-shaped parts452 and 453 (or the combined system of the support plate 451 and thesector-shaped parts 452 and 453) will have a reduced temperaturedependence regarding the change of polarization, since the temperaturedependence of the polarization state becomes partly compensated. Thisadvantageous effect is explained in more detail below with respect toFIG. 11.

Furthermore, although in the embodiment as shown in FIGS. 4 g and 4 hthe thicknesses of the sector-shaped parts 452 and 453 and the supportplate 451 are such that the orientation of polarization of linearlypolarized light with normal incidence on the light entrance side of thepolarization-modulating element 450 is rotated by an angle of 90° in thesector-shaped parts 452 and 453, and by an angle of 180° in the supportplate 451, the invention is not limited thereto. In an alternateembodiment, the thicknesses d₁, d₂ of the sector-shaped parts 452 and453 and the support plate 451 may e.g. be such that the orientation ofpolarization of linearly polarized light with normal incidence on thelight entrance side of the polarization-modulating element 450 isrotated by an angle of 180° in the sector-shaped parts 452 and 453, andby an angle of 90° (or more generally an angle of 90°+k*180°, with kbeing an integer≧0) in the support plate 451. In a further, more generalembodiment, the thickness of the support plate 451 may be such that theorientation of polarization of linearly polarized light with normalincidence on the light entrance side of the polarization-modulatingelement 450 is rotated by an angle of k*90°.

In FIG. 4 g, the directions of polarization of light that exits from thepolarization-modulating element 450 (and which has entered thepolarization-modulating element 450 as linear polarized light withpolarization along the y-axis) are referenced with double-arrows “P1” to“P4”. It can be seen that the embodiment as shown in FIGS. 4 g and 4 hmay be used to approximate a tangential polarization direction for theexiting light bundle. However, although in the embodiment according toFIGS. 4 g and 4 h the polarization-modulating element 450 comprises twosector-shaped parts 452 and 453 being arranged on the support plate 451,the invention is not limited to the shapes and areas and the number ofsector-shaped parts. Other suitable shapes (having for example but notlimited to trapeze-shaped, rectangular, square, hexagonal or circulargeometries) as well as more or less sector-shaped parts can be used inthe embodiment of FIGS. 4 g and 4 h in order to create a desiredpolarization distribution.

FIG. 5 schematically illustrates how a polarization-modulating opticalelement 501 with a thickness profile according to FIGS. 3 and 4 dconverts the polarization distribution of an entering light bundle 513with a uniformly oriented linear polarization distribution 517 into atangential polarization 519 of an exiting light bundle 515. This can bevisualized as follows: A linearly polarized light ray of the enteringlight bundle 513 which traverses the polarization-modulating opticalelement at a location of minimum thickness, for example at θ=180°,covers a distance of 90°/a inside the optically active crystal. Thiscauses the oscillation plane of the electrical field vector to berotated by 90°. On the other hand, a linearly polarized light raytraversing the polarization-modulating optical element 501 at a locationwith θ=45° covers a distance of 135°/a inside the optically activecrystal, thus the oscillation plane of the electrical field vector ofthis ray is rotated by 135°. Analogous conclusions can be drawn for eachlight ray of the entering light bundle 513.

FIG. 6 schematically illustrates how an optical arrangement with apolarization-modulating optical element 601 with a thickness profileaccording to FIGS. 3 and 4 d in combination with a furtherpolarization-modulating element 621 converts the polarizationdistribution of an entering light bundle 613 with a uniformly orientedlinear polarization distribution 617 into a radial polarization 623 ofan exiting light bundle 615. As explained in the context of FIG. 5, thepolarization-modulating optical element 601 produces a tangentialpolarization distribution. A tangential polarization distribution can beconverted into a radial polarization distribution by a 90°-rotation ofthe respective oscillation plane of each individual linearly polarizedray of the light bundle. There are several different possibilities toaccomplish this with an optical arrangement according to FIG. 6. Onepossible concept is to arrange a planar-parallel plate of an opticallyactive crystal as a further polarization-modulating element 621 in thelight path, where the thickness of the plate is approximately 90°/α_(p)with α_(p) representing the specific rotation of the optically activecrystal. As in the polarization-modulating element 601, the opticalcrystal axis of the planar parallel plate runs likewise parallel to theelement axis. As another possible concept, the furtherpolarization-modulating element 621 can be configured as a 90°-rotatorthat is assembled from two half-wave plates. A 90°-rotator consists oftwo half-wave plates of birefringent crystal material. Each plate has aslow axis associated with the direction of the higher refractive indexand, perpendicular to the slow axis, a fast axis associated with thedirection of the lower refractive index. The two half-wave plates arerotated relative to each other so their respective fast and slow axesare set at an angle of 45° from each other.

Of course further possible embodiments for producing a radialpolarization distribution are conceivable within the scope of theinvention. For example, the further polarization-modulating opticalelement 621 can be connected to the polarization-modulating opticalelement 601. To allow a fast change-over from tangential to radialpolarization, one could provide an exchange device that allows thefurther polarization-modulating element 621 to be placed in the lightpath and to be removed again or to be replaced by another element.

A tangential polarization distribution can also be produced with apolarization-modulating optical element that has a thickness profile inaccordance with FIG. 4 e. The thickness profile in this embodiment ofthe invention has no discontinuities. As visualized in FIG. 7 a, theuniformly oriented polarization distribution 717 of the entering lightbundle 713 is first transformed by the polarization-modulating opticalelement 701 into a linear polarization distribution 727 of an exitinglight bundle 715. The one-half of the entering light bundle 713 thatpasses through the polarization-modulating optical element 701 in theazimuth range 0≦θ≦180° of the thickness profile shown in FIG. 4 e isconverted so that the corresponding one-half of the exiting light bundlehas a tangential polarization distribution. The other half, however, hasa different, non-tangential polarization distribution 727. A furtherpolarization-modulating optical element is needed in the light path inorder to completely convert the polarization distribution 727 of thelight bundle 715 exiting from the polarization-modulating opticalelement 701 into a tangential polarization distribution 719. The furtherpolarization-modulating optical element is in this case configured as aplanar-parallel plate 725 with a first half 729 and a second half 731. Aplan view of the planar-parallel plate 725 is shown in FIG. 7 b. Thefirst half 729 is made of an isotropic material that has no effect onthe state of polarization of a light ray, while the second half 731 isdesigned as a half-wave plate. The planar-parallel plate 725 in theoptical arrangement of FIG. 7 a is oriented so that a projection RA′ ofthe reference axis RA of the polarization-modulating optical element 701onto the planar-parallel plate runs substantially along the separationline between the first half 729 and the second half 731. The slow axisLA of the birefringence of the half-wave plate is perpendicular to thisseparation line. Alternatively tangential polarization can also beachieved with a polarization-modulating optical element, having athickness profile as given by FIG. 4 e, if the element is composed oftwo half wedge-shaped or helically shaped elements of crystallinequartz, wherein the optical activity of one element is clockwise andthat of the other is counterclockwise. In this case no additionalplane-parallel plate 725 is necessary, as it is in the embodiment ofFIG. 7 a. In this embodiment preferably each wedge-shaped element has aconstant screw-slope, but the slopes have different directions as shownin the profile of FIG. 4 e. Further, it is not necessary that the slopesof the geometrical thickness d have the same absolute values, it issufficient if the slopes D of the optical effective thicknesses have thesame absolute values. In this case the specific rotations α aredifferent regarding absolute values for the two wedge-shaped elementswhich form the polarization-modulating optical element.

FIG. 8 schematically illustrates a microlithography projection system833 which includes the light source unit 835, the illumination system839, the mask 853 which carries a microstructure, the projectionobjective 855, and the substrate 859 that is being exposed to theprojection. The light source unit 835 includes a DUV- or VUV-laser, forexample an ArF laser for 192 nm, an F₂ laser for 157 nm, an Ar₂ laserfor 126 nm or a Ne₂ laser for 109 nm, and a beam-shaping optical systemwhich produces a parallel light bundle. The rays of the light bundlehave a linear polarization distribution where the oscillation planes ofthe electrical field vectors of the individual light rays are orientedin a uniform direction. The principal configuration of the illuminationsystem 839 is described in DE 195 20 563 A1 (U.S. Pat. No. 6,285,433B1). The parallel light bundle falls on the divergence-increasingoptical element 837. As a divergence-increasing optical element, onecould use for example a raster plate with an arrangement of diffractiveor refractive raster elements. Each raster element generates a lightbundle whose angle distribution is determined by the dimension and focallength of the raster element. The raster plate is located in or near theobject plane of an objective 840 that follows downstream in the lightpath. The objective 840 is a zoom objective which generates a parallellight bundle with a variable diameter. A direction-changing mirror 841directs the parallel light bundle to an optical unit 842 which containsan axicon (i.e., a rotationally symmetric prism arrangement) 843. Thezoom objective 840 in cooperation with the axicon 843 generatesdifferent illumination profiles in the pupil plane 845, depending on thesetting of the zoom and the position of the axicon elements. Apolarization-modulating optical element 801, for example of the kindshown in FIG. 3, is arranged in the pupil plane 845. Thepolarization-modulating optical element 801 is followed in the lightpath by a compensation plate 847 which has a thickness profile designedto compensate the angle deviations which the polarization-modulatingoptical element causes in the light rays that pass through it. Theoptical unit 842 is followed by a reticle-masking system (REMA) 849. TheREMA Objective 851 projects an image of the reticle-masking system 849onto the structure-carrying mask (reticle) 853, whereby the illuminatedarea of the reticle 853 is delimited. The projection objective 855projects the image of the structure-carrying mask 853 onto thelight-sensitive substrate 859. The space between the last opticalelement 857 of the projection objective and the light-sensitivesubstrate 859 contains an immersion liquid 861 with a refractive indexdifferent from air.

An additional advantage of the present invention is thatpolarization-modulating optical elements or the optical system accordingto the present invention can be used for adjusting the polarizationdistribution and also for temperature compensation of the polarizationdistribution in a microlithography projection system as described inFIG. 8. Advanced microlithography projection systems require in someapplications a predefined polarization distribution at the reticle 853with an accuracy of about 5° or even better, in some cases even betterthan 1°.

Since the polarization distribution at the reticle is influenced by thevarious optical elements by e.g. tension-induced birefringence, or byundefined or uncontrolled changes of the temperature of individualoptical elements, the polarization distribution can unpredictably oruncontrollably change over time. To correct such changes the temperaturedependency of the specific rotation a of the polarization-modulatingoptical element can be used to control the magnitude of the polarizationangles. The optical system according to an embodiment of the presentinvention preferably comprises a polarization control system forcontrolling the polarization distribution of the light beam which ispropagating through the optical system. The polarization distribution ofinterest is at a predefined location in the optical system. Thepolarization control system comprises at least one heating or coolingdevice to modify the temperature and/or the temperature distribution ofthe polarization-modulating optical element to affect the polarizationdistribution of the light beam at the predefined location. Here thepolarization-modulating optical element may have a varying or constanteffective optical thickness.

In the case of a constant effective optical thickness the optical systemcomprises an optical axis or a preferred direction given by thedirection of a light beam propagating through the optical system. Theoptical system additionally comprises a polarization-modulating opticalelement described by coordinates of a coordinate system, wherein onepreferred coordinate of the coordinate system is parallel to the opticalaxis or parallel to said preferred direction. Thepolarization-modulating optical element comprises solid and/or liquidoptically active material, wherein the effective optical thickness isconstant as a function of at least one coordinate different from thepreferred coordinate of the coordinate system. The optical systemcomprises further a polarization control system for controlling thepolarization distribution of the light beam (propagating through theoptical system) at a predefined location in the optical system, and thepolarization control system comprises at least one heating or coolingdevice to modify the temperature and/or the temperature distribution ofthe polarization-modulating optical element to affect the polarizationdistribution of the light beam at the predefined location.

As an example, if the polarization-modulating optical element (as usede.g. in the optical system according to the present invention) is madeof synthetic (crystalline) quartz, comprising a parallel plate or formedas a parallel plate, a thickness of 10 mm of such a plate will result ina change of polarization of 23.6 mrad/° C. or 23.6 mrad/K, equivalent to1.35°/K, due to the linear temperature coefficient γ of the specificrotation α with γ=2.36 mrad/(mm*K). These data correspond to awavelength of 193 nm. In such an embodiment, which is schematicallyshown in FIG. 9, the optical axis OA of the parallel plate 901 isdirected parallel or approximately parallel to the propagation of thelight (indicated by reference numeral 950) in the optical system.Approximately parallel means that the angle between the optical axis OAof the parallel plate 901 and the direction of the light propagatingthrough the optical system is smaller than 200 mrad, preferably smallerthan 100 mrad or even smaller than 50 mrad. Controlling the temperatureof the plate 901 will result in a controlled change of polarization. Iffor example the temperature of the plate will be controlled in a rangeof about 20° C. to 40° C., the polarization angles can be controllablychanged in a range of about ±13.5° for such a plate 901 made of quartz.This high sensitivity allows a control of the polarization distributionby temperature control. In such a case even a plane plate with athickness d of about 0.1 mm up to 20 mm will become apolarization-modulating optical element 901, able to controllably adjusta polarization distribution by controlling the temperature of the plate901. Preferably for synthetic (crystalline) quartz the thickness of theplate 901 is n*278.5 μm (n is any integer) which results in a rotationof a polarization plane of at least 90° for n=1 and 180° for n=2 and ingeneral n*90°, for a wavelength of 193 nm at about 21.6° C. For a 90°rotation of the polarization plane the synthetic quartz should be atleast 278.5 μm thick and for 180° at least 557.1 μm, for 270° thethickness should be 835.5 μm and for a 360° rotation of the polarizationthe thickness is 1.114 mm. The manufacturing tolerances regardingthickness are about ±2 μm. Thus the manufacturing tolerance results inan inaccuracy of the angle of the polarization plane of the light whichpasses the plate of about ±0.64° at about 21.6° C. and 193 nm. To thisinaccuracy an additional inaccuracy caused by temperature fluctuation ofthe plate (or polarization-modulating optical element) have to beconsidered, which is given by the linear temperature coefficient γ ofthe specific rotation a with is γ=2.36 mrad/(mm*K)=0.15°/(mm*K).

The temperature control of the plate 901 can be done by closed-loop oropen-loop control, using a temperature sensing device with at least onetemperature sensor 902, 903 for determining the temperature of the plate901 (or providing a temperature sensor value which is representative orequal to the temperature and/or the temperature distribution of thepolarization-modulating optical element), at least a heater 904, 905,preferably comprising an infrared heater, for heating the plate byinfrared radiation 906, and a control circuit 910 for controlling the atleast one heater 904, 905. As an example of a temperature sensing devicea infrared sensitive CCD-element with a projection optics may be used,wherein the projection optics images at least a part of the plate 901onto the CCD-element such that a temperature profile of the viewed partof the plate 901 can be determined by the analysis of the CCD-elementsignals. The control circuit 910 may comprise a computer system 915 ormay be connected to the computer or control system 915 of themicrolithography projection system 833 (see FIG. 8). In a preferredembodiment of the temperature controlled plate 901 the thickness ischosen such that a rotation of the polarization of n*90°, n is anyinteger number, is achieved at a temperatureT=(T_(max)−T_(min))/2+T_(min), whereas T_(max) and T_(min), are themaximum and minimum temperatures of the plate 901 (or in general thepolarization-manipulating optical element). Preferably the heater orheating system (and also any cooling device like a Peltier element) isarranged such that it is not in the optical path of the microlithographyprojection system 833, or that it is not in the optical path of thelight beam which is propagating through the optical system according toan embodiment of the present invention. Preferably the optical systemwith the polarization control system according to the present inventionis used in a system with at least one additional optical elementarranged between the polarization-modulating optical element and thepredefined location in the optical system such that the light beamcontacts the at least one additional optical element when propagatingfrom the polarization-modulating optical element to the predefinedlocation. The additional optical element preferably comprises a lens, aprism, a mirror, a refractive or a diffractive optical element or anoptical element comprising linear birefringent material. Thus theoptical system according to the present invention may form a part of amicrolithography projection system 833.

In a further preferred embodiment the temperature of thepolarization-manipulating optical element 901 (the plate as shown inFIG. 9) corresponds to a predefined temperature profile. As an example,such a temperature profile is achieved by using a plurality of infraredheaters 904, 905 to produce a radiation distribution across the opticalelement 901 which heats the optical element 901 in a controlled way witha control circuit as already described. In such an embodiment also aplurality of temperature sensors 902, 903 can be used for the controlcircuit 910. With this embodiment the polarization state in a fieldplane or pupil plane of the microlithography projection system 833 canbe adjusted locally.

Alternatively or in addition the heater or heating elements 904, 905 maybe replaced or supplemented by one or more Peltier-elements 907, 908.The Peltier-element or elements are preferably connected to the controlcircuit 910 such that a control by open and/or closed loop control ispossible. The advantage of the Peltier-elements is that also acontrolled cooling of the polarization-manipulating optical element 901can be achieved. Heating and cooling the optical element 901 at the sametime result in complex temperature distributions in thepolarization-modulating optical element 901, which result in complexpolarization distributions of the light 950 propagating e.g. through themicrolithography projection system 833, after passing the element 901.Of course, other heating and cooling means than the ones mentioned abovecan be used to achieve a required temperature profile or a requiredtemperature of the polarization-modulating optical element 901.

The application of the plane plate 901 as polarization-modulatingoptical element 801 in the illumination system of a microlithographyprojection apparatus 833 (see FIG. 8) is preferably in the pupil plane845 and/or at positions between the light source unit 835 and thementioned pupil plane 845. Applying the plane plate 901 at theselocations has the advantage that the angle of incidence of the lightwhich passes through the plate 901 and also passing through themicrolithography projection apparatus is smaller than about 6° (100mrad). At these small angles the influence of linear birefringence,caused by the plate 901, is very small such that the polarization of thelight after passing the plate 901 is almost linear with negligibleelliptical parts, if the light was linearly polarized before enteringthe plate 901.

In a further preferred embodiment of the invention the state of thepolarization of the light passed through the polarization-modulatingelement 901 or the optical system according to the present invention ismeasured. For this the polarization control system comprises apolarization measuring device providing a polarization valuerepresentative for or equal to the polarization or the polarizationdistribution of the light beam at the predetermined location in theoptical system. Further, the control circuit controls the at least oneheating or cooling device dependent on the temperature sensor valueand/or the polarization value by open or closed loop control. Themeasured state of polarization is compared with a required state and inthe case that the measured state deviates more than a tolerable value,the temperature and/or the temperature distribution of thepolarizing-modulating element like the plane plate 901 is changed suchthat the difference between the measured and the required state ofpolarization becomes smaller, and if possible such small that thedifference is within a tolerable value. In FIG. 9 the measurement of thestate of polarization is measured in-situ or with a separate specialmeasurement, depending on the polarization measuring device 960. Thepolarization measuring device may be connected with the control circuit910, such that depending on the measured polarization state values theheating means 904, 905 and/or 907, 908 are controlled heated and/orcooled such that the measured and the required state of polarizationbecomes smaller. The control can be done in open or closed loop modus.

The plane plate 901 used as polarization-modulating optical element orbeing a part of such element is especially appropriate to correctorientations of polarization states of the passed light bundles.

In a further embodiment of the present invention the plane plate 901(comprising or consisting of optically active material), used as apolarization-modulating optical element, is combined with a plate 971(see FIG. 10), comprising or consisting of linear birefringent material.With this embodiment of the invention the orientation and the phase ofthe passing light bundle 950 can be subjected such that e.g. a planepolarized light bundle becomes elliptically polarized after passing bothplane plates 901 and 971, or vice versa. In this embodiment at least oneplate 901 or 971 is controlled regarding its temperature and/ortemperature distribution as described in connection with FIG. 9.Further, the sequence of the plates 901 and 971 may be changed such thatthe passing light bundles are first passing through the plate 971,comprising or consisting of linear birefringent material, and thanthrough the plate 901, comprising or consisting of optical activematerial, or vice versa. Preferably both plates are consecutivelyarranged along the optical axis OA of the system. Also, more than oneplate comprising or consisting of linear birefringent material, and/ormore than one plate comprising or consisting of optical active materialmay be used to manipulate the state of polarization of the passing lightbundles. Further, a plane plate 971, or 901 may be exchanged by a liquidcell or cuvette containing optically active material. Also the planeplates 971, comprising or consisting of linear birefringent material,and plate 901, comprising or consisting of optical active material, canbe arranged such that at least one other optical element 981 is placedbetween these plane plates. This element 981 can be for example a lens,a diffractive or refractive optical element, a mirror or an additionalplane plate.

In an additional embodiment of the present invention apolarization-modulating element or in general a polarizing opticalelement is temperature compensated to reduce any inaccuracy of thepolarization distribution generated by the polarization-modulatingelement due to temperature fluctuations of said element, which forsynthetic quartz material is given by the linear temperature coefficientγ of the specific rotation a for quartz (which is as already mentionedabove γ=2.36 mrad/(mm*K)=0.15°/(mm*K)). The temperature compensationmakes use of the realization that for synthetic quartz there exist onequartz material with a clockwise and one quartz material with acounterclockwise optical activity (R-quartz and L-quartz). Both, theclockwise and the counterclockwise optical activities are almost equalin magnitude regarding the respective specific rotations a. Thedifference of the specific rotations is less than 0.3%. Whether thesynthetic quartz has clockwise (R-quartz) or counterclockwise (L-quartz)optical activity dependents on the seed-crystal which is used in themanufacturing process of the synthetic quartz.

R- and L-quartz can be combined for producing a thermal or temperaturecompensated polarization-modulating optical element 911 as shown in FIG.11. Regarding the change of the state of polarization such a temperaturecompensated polarization-modulating optical element 911 is equivalent toa plane plate of synthetic quartz of thickness d. For example, two planeplates 921 and 931 are arranged behind each other in the direction 950of the light which is propagating through the optical system whichcomprises the temperature compensated polarization-modulating opticalelement 911. The arrangement of the plates is such that one plate 931 ismade of R-quartz with thickness d_(R), and the other 921 is made ofL-quartz with thickness d_(L), and |d_(R)−d_(L)|=d. If the smallerthickness of d_(R) and d_(L) (min(d_(R), d_(L))) is larger than d ormin(d_(R), d_(L))>d, which in most cases is a requirement due tomechanical stability of the optical element, then the temperaturedependence of the polarization state becomes partly compensated, meaningthat the temperature dependence of the system of R-quartz and L-quartzplates is smaller than γ=2.36 mrad/(mm*K)*d=0.15°/(mm*K)*d, wherein d isthe absolute value of the difference of the thicknesses of the twoplates d=|d_(R)−d_(L)|. The following example demonstrates this effect.A R-quartz plate 931 with a thickness of e.g. d_(R)=557.1 μm (resultingin a 180° clockwise change of the exiting polarization plane compared tothe incident polarization plane) is combined with a L-quartz plate 921with a thickness of d_(L)=557.1 μm+287.5 μm (resulting in a 270°counterclockwise change of the exiting polarization plane compared tothe incident polarization). This result in a 90° counterclockwise changeof the polarization plane after the light pass both plane plates 921,931, corresponding to a 270° clockwise change of the polarization planeif just a R-quartz plate would be used. In this case the temperaturecompensation is not fully achieved, but it is reduced to value of about0.04°/K if both plates are used, compared to 0.13°/K if just a R-quartzplate of d_(R)=557.1 μm+287.5 μm would be used. This is a significantreduction of temperature dependency, since even if the temperature willchange by 10° C. the change of the polarization plane is still smallerthan 1°.

In general any structured polarization-modulating optical element madeof R- or L-quartz, like e.g. the elements as described in connectionwith FIGS. 3 and 4 a can be combined with a plane plate of therespective other quartz type (L- or R-quartz) such that the combinedsystem 911 will have a reduced temperature dependence regarding thechange of the polarization. Instead of the plane plate also a structuredoptical element made of the respective other quartz type may be usedsuch that in FIG. 11 the shown plates 921 and 931 can be structuredpolarization-modulating optical elements as mentioned in thisspecification, having specific rotations of opposite signs, changing thestate of polarization clockwise and counterclockwise.

To generalize the above example of a temperature compensatedpolarization-modulating optical element 911, the present invention alsorelates to an optical system comprising an optical axis OA or apreferred direction 950 given by the direction of a light beampropagating through the optical system. The optical system comprising atemperature compensated polarization-modulating optical element 911described by coordinates of a coordinate system, wherein one preferredcoordinate of the coordinate system is parallel to the optical axis OAor parallel to said preferred direction 950. The temperature compensatedpolarization-modulating optical element 911 comprises a first 921 and asecond 931 polarization-modulating optical element. The first and/or thesecond polarization-modulating optical element comprising solid and/orliquid optically active material and a profile of effective opticalthickness, wherein the effective optical thickness varies at least as afunction of one coordinate different from the preferred coordinate ofthe coordinate system. In addition or alternative the first 921 and/orthe second 931 polarization-modulating optical element comprises solidand/or liquid optically active material, wherein the effective opticalthickness is constant as a function of at least one coordinate differentfrom the preferred coordinate of the coordinate system. As an additionalfeature, the first and the second polarization-modulating opticalelements 921, 931 comprise optically active materials with specificrotations of opposite signs, or the first polarization-modulatingoptical element comprises optically active material with a specificrotation of opposite sign compared to the optically active material ofthe second polarization-modulating optical element. In the case of planeplates, preferably the absolute value of the difference of the first andthe second thickness of the first and second plate is smaller than thethickness of the smaller plate.

In an additional embodiment of the present invention apolarization-modulating element comprises an optically active and/oroptically inactive material component subjected to a magnetic field suchthat there is a field component of the magnetic field along thedirection of the propagation of the light beam through thepolarization-modulating element. The optical active material componentmay be constructed as described above. However, also optical inactivematerials can be used, having the same or similar structures asdescribed in connection with the optical active materials. Theapplication of a magnetic field will also change the polarization stateof the light passing through the optical active and/or optical inactivematerial due to the Faraday-effect, and the polarization state can becontrolled by the magnetic field.

In the following, further embodiments of the present invention relatedto the above-described aspect of temperature-compensation are describedwith reference to FIGS. 12 to 16.

FIG. 12 also shows an arrangement 10 of two plane plates 11 and 12 beingarranged behind each other along the optical axis oa of an opticalsystem and made of optically active material (crystalline quartz). Morespecifically, plate 11 is made from optically active quartz with aspecific rotation of opposite sign compared to the optically activequartz of the second plate 12. In the illustrated example, the firstplate 11 is made of right-handed quartz, whereas the second plate 12 ismade of left-handed quartz (which can of course be vice-versa, too). Incontrast to FIG. 11, the thicknesses of plates 11 and 12 are identicaland may e.g. be (in a non-limiting example) 0.5 mm. In order to realizethe principle of the embodiment shown in FIG. 12, the thicknesses d1, d2of the plates 11 and 12 just have to be substantially identical, e.g. bymeeting the criterion |d1−d2|≦0.01*(d1+d2). As a consequence, if thefirst, right-handed plate 11 rotates the orientation of the polarizationof linear polarized light by an excess of e.g. 5.9 mrad (i.e. by 5.9mrad too much) as a result of a temperature shift of Δ5K, this effect iscompensated since the second left-handed plate 12 rotates theorientation of the polarization of linear polarized light by atoo-little amount of 5.9 mrad.

As a consequence, the temperature effects in both plates 11, 12compensate each other, so that—due to the substantially identicalthicknesses of the plates 11, 12—the polarization state of light passingboth plates 11, 12 remains unchanged. The arrangement shown in FIG. 12may in particularly be used to provide a diffractive optical element, asit is just schematically illustrated in FIG. 13. Hereto, a diffractivestructure 13 can be applied e.g. on the first plate 11. Since such astructure 13 typically has a depth of the etched structures in the rangeof 200-400 nm, which is typically more than three orders of magnitudelower than the typical thicknesses of the first and second plate,respectively, the contribution of the structure 13 to a change of thecircular birefringence can be neglected. With the arrangements shown inFIGS. 12 and 13, a detrimental effect of temperature variations on thepolarization state of light passing therethrough can be avoided evenwith relatively large thicknesses (e.g. of several mm) of the plates 11,12, which qualifies the arrangement of the plates for use as a supporte.g. of a diffractive optical element as shown in FIG. 13. Furthermore,due to the use of crystalline material instead of e.g. amorphous glassor fused silica, the undesired effect of thermal compaction leading tolocal variations of the density and non-deterministic birefringenceproperties, which would inadvertently modify or destroy the polarizationdistribution after the optical arrangement, is avoided or significantlyreduced even in the presence of electromagnetic radiation of relativelyhigh energies.

With reference to FIGS. 14 a and 14 b, further arrangements aredescribed which enable, in addition to the above described temperaturecompensation, also the use of the respective embodiment in a region ofrelatively large aperture angles without occurrence of a significantundesired modification of the polarization distribution after therespective arrangement.

FIG. 14 a shows a single plane plate 25 being made of right-handed (oralternatively left-handed) optically active quartz such that the opticalcrystal axis is aligned with the direction of optical system axis oa(z-axis). Furthermore, the thickness of plate 25 is selected such thatthe orientation of the direction of linear polarized light whichperpendicularly enters the plate 25 is rotated by an angle of 180° or aninteger multiple thereof. If synthetic, optically active quartz is usedwhich has at a wavelength of 193 nm a specific rotation of α=323.1°/mmat a temperature of 21.6° C., this condition corresponds for a singlerotation of 180° to a thickness of ≅557 μm.

As to the light beam “1” which passes plate 305 along the opticalcrystal axis, only circular birefringence and no linear birefringenceoccurs. As to light beam “2” which passes plate 25 not parallel to theoptical crystal axis, the additional effect of linear birefringenceoccurs, with said linear birefringence reaching its maximum value if thelight beam is perpendicular to the optical crystal axis, whereas theeffect of circular birefringence decreases for increasing angularbetween the light beam and the optical crystal axis. Since theorientation of polarization is rotated between the light entrancesurface and the light exit surface of plate 25 by ≅180° (which isapproximately true also for beam “2” in spite of the decreasing circularbirefringence if also considering the increased travelling path), saidorientation of polarization is rotated by ≅90° after beam “2” has passedhalf of the thickness of plate 25. As a consequence, a polarityinversion (i.e. a reversal of the signs) occurs for the linearbirefringence after beam “2” has passed half of the thickness of plate25, so that the phase shifts collected due to linear birefringence whilepassing the first half of plate 25 are corrected back by the phaseshifts collected due to linear birefringence while passing the secondhalf of plate 25, which means that the effect of linear birefringence isalmost nearly compensated for beam “2” after having passed the wholeplate 25.

FIG. 14 b shows an arrangement similar to FIG. 12 in so far as anarrangement 30 comprises two plane plates 31 and 32 being arrangedbehind each other along the optical axis oa of an optical system andmade of optically active material (crystalline quartz). Plate 31 is madefrom left-handed optically active quartz, i.e. with a specific rotationof opposite sign compared to the right-handed optically active quartz ofthe second plate 32. The thicknesses d1 and d2 of plates 31 and 32 areat least substantially identical, e.g. by meeting the criterion|d1−d2|≦0.01*(d1+d2). Furthermore, the thicknesses d1 and d2 of plates31 and 32 are selected such that the orientation of the direction oflinear polarized light which perpendicularly enters plate 32, or plate31 respectively, is rotated, in each of the plates 31 and 32, by anangle of 180° or an integer multiple thereof. If synthetic, opticallyactive quartz is used which has at a wavelength of 193 nm a specificrotation of α=323.1°/mm at a temperature of 21.6° C., this conditioncorresponds for a single rotation of 180° in each plate 31, 32 to athickness of d1=d2≅557 μm. More generally, the thicknesses d1 and d2 ofplates 31 and 32 are selected such that the orientation of the directionof linear polarized light which perpendicularly enters plate 32, orplate 31 respectively, is rotated, in each of the plates 31 and 32, forthe operating wavelength λ (of e.g. 193 nm) by an angular in the regionof 160°+N*180° to 200°+N*180°, more preferably in the region of170°+N*180° to 190°+N*180°, still more preferably by an angular ofN*180° (with N being an integer greater or equal zero). This arrangementalso results in a relatively weak sensitivity of the polarization stateon the angle of incidence of the beam as explained in the following.

As to the light beam “1” which passes plates 32 and 31 along the opticalcrystal axis, only circular birefringence and no linear birefringenceoccurs. The orientation of polarization of beam “1” is rotated clockwisein plate 32 by 180° and is then rotated counterclockwise in plate 31 by180°, so that the orientation of polarization beam 1 is effectively notrotated when passing the whole arrangement of plates 32 and 31. Lightbeam “2” enters plate 32 under an angle of incidence larger than zeroand therefore experiences also the effect of linear birefringence,whereas the effect of circular birefringence decreases with increasingangle of incidence. As a consequence, the orientation of polarizationfor light beam 2 is rotated clockwise in plate 32 by less than 180° (seediagram 2 b on the right side of FIG. 14 b), and the light beam “2” willbe weakly elliptically polarized after exit of plate 32. However, due tothe above selection of thickness of plate 32, the effect of linearbirefringence is almost compensated so that said elliptical portion isrelatively weak. Since the first plate 31 is passed with oppositerotational direction and the same travelling distance as the secondplate 32, the orientation of polarization of light beam “2” is rotatedback by substantially the same angle when passing the second plate 32(see diagram 2 c on the right side of FIG. 14 b). Accordingly, theorientation of polarization of light beam “2” upon exit of plate 31 issubstantially identical to the orientation of polarization of light beam“2” upon entrance in plate 32 (see diagrams 2 a and 2 c on the rightside of FIG. 14 b). Furthermore, since the decrease of the opticalactivity with increasing angle of incidence is substantially equal inboth plates 31 and 32, said effect is not disturbing the maintenance ofthe polarization state.

As described above with reference to FIG. 14 a, the relatively weaksensitivity of the polarization state on the angle of incidence of thebeam is also achieved for only one plane plate 25 whose thickness isselected such that the orientation of the direction of linear polarizedlight which perpendicularly enters the plate 25 is rotated by an angleof 180° or an integer multiple thereof. However, due to the combinationof two plates 31 and 32 according to FIG. 14 b, i.e. with specificrotations of opposite sign compared to each other, a detrimental effectof temperature variations on the polarization state of light passingthrough the arrangement can be avoided even with relatively largethicknesses (e.g. of several mm) of the plates 31, 32, as alreadydescribed above. As to this effect of temperature compensation which isalso achieved for the embodiment of FIG. 14 b, reference can be made tothe embodiments described above with respect to FIGS. 12 and 13.

In the embodiment of FIG. 15, two plates as described above with respectto FIG. 12 (i.e. of right-handed or left-handed quartz, respectively,are combined (e.g. by optical wringing) to form a support 41 of a greyfilter 40, said grey filter 40 also comprising an absorbing structure 42of material having a reduced transmittance at the operating wavelengthof e.g. 193 nm. This structure can e.g. be formed of vapour-depositedchrome of variable thickness (e.g. in the region of 200-400 nm) anddensity.

In the embodiment of FIG. 16, a diffractive optical element 50 comprisesa support 51, which may be formed identical as the support 40 of FIG. 15but comprises a deepened structure 52. The support 51 can be madeaccording to anyone of the embodiments described with respect to FIG.12-14, i.e. by combining (preferably by wringing) two plane plates ofidentical thickness, wherein said thicknesses may be arbitrary butparticularly may be selected such that a rotation of the orientation ofpolarization of linear polarized light that perpendicularly enters theplates is rotated by an angle of substantially 180° in each of theplates. Preferably, the maximum depth of the structure 52 is not morethan 0.1% of the thickness of the support 51 (e.g. the maximum depth ofthe structure 52 is not more 1 μm if the thickness of the support 51 is1 mm), so that the compensation effect achieved by said 180°-rotation isachieved to a large extend. According to a further embodiment, theoptical element illustrated in FIG. 16 can also be used as a diffusionpanel.

As a consequence of the weak sensitivity of the grey filter 40 or thediffractive optical element 50, said micro-optical elements are suitableto be used at a position in an illumination system where relativelylarge energy densities occur, since at such positions the avoidance ofthe above-discussed compaction effects and the compensation of theeffects of temperature variations are particularly relevant. As anexample, an optical arrangement or an optical element according toanyone of the embodiments described with reference to FIGS. 12, 13, 14 band 15 can be arranged at a position in an illumination system where theenergy density of the illumination light is more than 130%, moreparticularly at least 200%, and still more particularly at least 300% ofthe energy density in the reticle plane of the illumination system.Further preferred positions of such an optical arrangement or elementare in the region of relatively large numerical aperture, e.g. at ornear the position of a filed defining element in the pupil plane (seereference number 845 in FIG. 8). Preferred positions are furthercharacterized in that the maximum aperture angle of a light beamrelative to the optical axis at the respective position is not more than350 mrad or the numerical aperture is not more than 0.4. A furtherembodiment is a diffusion panel arranged downstream of the pupil plane(see reference number 845 in FIG. 8). As a consequence of the relativelyweak sensitivity of the polarization state on the angle of incidence ofthe beam for the embodiment of FIG. 14 b, the maximum aperture angle ofa light beam relative to the optical axis at the respective position ofsuch an optical arrangement or element can be at least 200 mrad, moreparticularly more than 300 mrad. Such an optical arrangement or elementis also advantageously used in an illumination system in combinationwith certain illumination settings providing relatively large apertureangles (such as annular illumination, dipole or quadrupoleillumination).

Going back to the embodiments of the polarization-modulating opticalelement discussed before and having a variable thickness profilemeasured in the direction of the optical crystal axis, furtheradvantageous embodiments are described in the following with referenceto FIG. 17 a-d and 18 a-c.

As already mentioned above, the refraction occurring in particular atsloped surfaces of the polarization-modulating element can cause adeviation in the direction of an originally axis-parallel light rayafter it has passed through the polarization-modulating element. Inorder to compensate for this type of deviation, it is advantageous toarrange a compensation plate in the light path of an optical system,with a thickness profile of the compensation plate designed so that itsubstantially compensates an angular deviation of the transmittedradiation that is caused by the polarization-modulating optical element.In an advantageous embodiment of the present invention which isexplained in more detail in the following, said compensation plate isalso made of an optically active material.

FIGS. 17 a-d shows different views on a combination of apolarization-modulating optical element with a compensation plateaccording to preferred embodiments of the present invention, while FIGS.18 a-c show thickness profiles as a function of the azimuth angle forvarious embodiments with different combinations of apolarization-modulating optical element with a compensation plate. Theview shown in FIG. 17 b is a cross-section along the dashed line indirection of the arrows “b”, whereas the views shown in FIGS. 17 c and17 d are side views on the arrangement in FIG. 17 a from the left (FIG.17 c) or the right (FIG. 17 d), respectively.

The optical arrangement according to FIG. 17 a-d comprises apolarization-modulating optical element 110 composed of parts 110 a and110 b, and a compensation plate 120 composed of parts 120 a and 120 b.Of course, the composition of the polarization-modulating opticalelement 110 and the compensation plate 120 is just for technologicalreasons and may principally also be omitted or replaced by a compositionof more than 2 parts. The optical element 110 further has an optionalcentral bore 130 coaxial with the element axis.

The shape of the polarization-modulating optical element 110 shown inFIG. 12 a-d corresponds to that of the polarization-modulating opticalelement 301 shown in FIG. 3, but is illustrated in FIG. 17 a-d toschematically explain the relative arrangement of a compensation plate120 in relation to the polarization-modulating optical element 110.Accordingly, the polarization-modulating optical element 110 shown inFIG. 17 a-d has a planar base surface and an opposite inclined surfacedesigned to achieve a thickness profile as already explained above withreference to FIG. 4 d. In the special configuration illustrated in FIG.17, the thickness of the polarization-modulating optical element 110 isconstant along a radius R that is perpendicular to the element axis,which again is parallel to the z-axis in the coordinate system alsoillustrated in FIG. 17 a-d. Thus, like in FIG. 3, the thickness profilein the illustrated embodiment of FIG. 17, which is shown in FIG. 18 b,only depends on the azimuth angle θ and is given by d=d (θ). In anotherembodiment of the polarization-optical element the thickness of thepolarization-modulating optical element 110′ may vary along the radius Rsuch that the thickness profile is d=d(R,θ).

In the exemplarily embodiment, the polarization-modulating opticalelement 110 may be made of R-quartz, with the optical axis of theoptically active crystal running parallel to the element axis.“R-quartz” means that the optically active quartz is turning thedirection of polarization clockwise if seen through the optically activequartz towards the light source.

Furthermore, although the compensation plate 120 is shown, in FIG. 17,in front of the polarization-modulating optical element with respect tothe direction of light propagation (which is running into thez-direction), it can of course also be arranged behind thepolarization-modulating optical element.

As can also be seen in FIG. 17, the compensation plate 120 beingarranged in the light path of the optical system has a thickness profilebeing a complement to the thickness profile of thepolarization-modulating optical element 110 in such a sense that thecompensation plate 120 and the polarization-modulating optical element110 effectively add up to a plan-parallel structure. FIG. 18 b shows,for the optical arrangement of FIGS. 17 a-d, the thickness profile ofboth the polarization-optical element 110 and the compensation plate120. In FIG. 18 b, the thickness profiles of the parts 110 a and 110 bof the polarization-optical element 110 being illustrated with solidlines are designated as C1 and C2, while the thickness profiles of theparts 120 a and 120 b of the compensation plate 120 being illustratedwith dashed lines are designated as D1 and D2.

In the exemplarily embodiment with the polarization-modulating opticalelement 110 being made of R-quartz, the compensation plate 120 ispreferably made of L-quartz. “L-quartz” means that the optically activequartz is turning the direction of polarization counter-clockwise ifseen through the optically active quartz towards the light source. Ofcourse the polarization-modulating optical element 110 can also be, viceversa, made of L-quartz, with the compensation plate 120 being made ofR-quartz. More generally, the compensation plate 120 comprises anoptically active material with a specific rotation of opposite signcompared to said first optically active material.

Furthermore, as already discussed before, the invention is not limitedto the use of quartz or generally to the use of crystalline materials,so that both the polarization-modulating optical element and thecompensation plate may also be replaced by one or more cuvettes ofappropriate shape which are comprising an optically active liquid. Infurther more generalized embodiments, as has been already describedabove with reference to FIGS. 3 and 4, the thickness profiles discussedwith reference to FIGS. 17 and 18 are not representing the geometricalthicknesses of the polarization-optical element or the compensationplate, respectively, but the profile represents an optical effectivethickness D as defined above.

To evaluate the effect of this thickness profile, it has to beconsidered that since the polarization-optical element 110 and thecompensation plate 120 are turning the direction of polarization oflinear polarized light into opposite directions, the relevant factor forthe net effect on each light ray traversing the arrangement thepolarization-optical element 110 and the compensation plate 120 parallelto the optical axis of each of these elements is the difference of thethicknesses d or optically effective thicknesses D being passed in theL-quartz or the R-quartz, respectively. Since this difference is justzero at the two crossing points of the solid lines C1, C2 with thedashed lines D1, D2, which occur for an azimuth angle of θ=90° as wellas for an azimuth angle of θ=270°, a linearly polarized light raypassing the arrangement under an azimuth angle of θ=90° or θ=270° willleave the arrangement with the same orientation of polarization. Thismeans that for a generation of a tangential polarization distribution asit has been explained above with reference to FIG. 5, the azimuth anglesof θ=90° or θ=270° represent the “new” reference angles where theorientations of polarization are left unchanged with respect to thepolarization distribution of the light entering the arrangement. Asfollows from the above, the arrangement of FIG. 17 of thepolarization-optical element 110 and the compensation plate 120 beingmade of optically active materials with a specific rotation of oppositesign has to be placed in a position being rotated by 90° if compared tothe position taken by the arrangement of FIG. 3.

For thicknesses 90°<θ<180°, the travelled distance in the R-quartz ofthe polarization-optical element is larger than the travelled distancein the L-quartz of the compensation plate, leading to a clockwisenet-rotation of the direction of polarization. For thicknesses 0°<θ<90°,the travelled distance in the L-quartz of the compensation plate islarger than the travelled distance in the R-quartz of thepolarization-optical element, leading to a counter-clockwisenet-rotation of the direction of polarization.

Since both the polarization-modulating optical element 110 and thecompensation plate 120 are rotating the direction of polarization intoopposite directions, the slopes in the respective thickness profiles ofthe compensation plate and the polarization-modulating element may bereduced for each of these elements, if compared to a situation whereonly the polarization-modulating optical element is made of opticallyactive material. More specifically and with reference to FIG. 18 b, atangential polarization distribution can be achieved with half the slopeof the lines C1 and C2 describing the inclined surface of thepolarization-modulating element (or D1 and D2, respectively, describingthe inclined surface of the compensation plate). Accordingly, thethickness of the polarization-optical element 110 in this embodiment is,like in the embodiment explained with reference to FIGS. 3 and 4 d, alinear function of the azimuth angle θ, but with half the slope, theabsolute value of which being |m|=180°/(2·α·π) over each of two rangesof 0<θ<180° and 180°<θ<360° (of course this slope is also valid for thecompensation plate 120).

A modification of the arrangement of FIG. 17 which is comparable to theembodiment explained above with reference to FIG. 4 c, but alsocomprises a compensation plate, is shown in FIG. 18 a, giving an exampleof a continuously varying thickness profile. The thickness d or opticaleffective thickness D of the azimuthal section of thepolarization-optical element (whose thickness profile is shown withsolid line A) in this embodiment shows a linear decrease over the wholeazimuth-angle range of 0≦θ≦360°. Again, the shape of the compensationplate (whose thickness profile is shown with dashed line B) is acomplement in the sense that it shows a linear increase over the wholeazimuth-angle range of 0≦θ≦360°. Further, as it is also the case for theembodiment explained with reference to FIG. 18 b, a tangentialpolarization distribution can be achieved with half the slope of theline A describing the inclined surface of the polarization-modulatingelement (or B describing the inclined surface of the compensation plate)if compared to FIG. 4 c. Accordingly, the absolute value of the slope ofline A or B, respectively, is |m|=180°/(2·απ) over an azimuth-anglerange of 0≦θ≦360°. Alternatively the slope can also be |m|=180°/(2α·π·r)where r is the radius of a circle being centred at the element axis EA.In this case the slope depends on the distance of the element axis, e.g.if the polarization-modulating optical element has a given constantscrew-slope (lead of a screw).

A further modification of the arrangement of FIG. 17 which is comparableto the embodiment explained above with reference to FIG. 4 e, but alsocomprises a compensation plate is shown in FIG. 18 c. The thickness d oroptical effective thickness D of the azimuthal section of thepolarization-optical element (whose thickness profile is shown withsolid lines E1 and E2) is in this case a linear function of the azimuthangle θ with a first slope m for 0<θ<180° and with a second slope n for180°<0<360°. The slopes m and n are of equal absolute magnitude but haveopposite signs. As explained above with reference to

FIG. 4 e, the concept of using opposite signs for the slope in the twoazimuth angle ranges avoids the occurrence of discontinuities in thethickness profile. Again, the shape of the compensation plate (whosethickness profile is shown with dashed lines F1 and F2) is a complementin the sense that it also shows a linear function of the azimuth angle θwith a first slope for 0<θ<180° and with a second slope for 180°<θ<360°,but with the first and second slope being opposite to the first slope mor the second slope n, respectively, for the azimuthal section of thepolarization-optical element. Again, a tangential polarizationdistribution can be achieved with half the slope of the lines E1, E2describing the inclined surface of the polarization-modulating element(or F1, F2 describing the inclined surface of the compensation plate) ifcompared to FIG. 4 e. Accordingly, the absolute value of the slopes oflines E1 or E2, respectively, are at a distance r from the element axis|m|=180°/(2·απ·r) and |n|=180°/(2·α·π·r).

Various embodiments for a polarization-modulating optical element or forthe optical systems according to the present invention are described inthis application. Further, also additional embodiments ofpolarization-modulating optical elements or optical systems according tothe present invention may be obtained by exchanging and/or combiningindividual features and/or characteristics of the individual embodimentsdescribed in the present application.

1. A system, comprising: an illumination system; a projection objective;and a temperature compensated polarization-modulating optical element inthe illumination system, the temperature compensatedpolarization-modulating optical element comprising: a firstpolarization-modulating optical element comprising an optically activematerial, the first polarization-modulating optical element having afirst specific rotation with a sign; and a secondpolarization-modulating optical element comprising optically activematerial, the second polarization-modulating optical element having asecond specific rotation with a sign opposite to the sign of the firstspecific rotation, wherein the system is a microlithography opticalsystem.